Even at its busiest, the biggest man-made lake in Europe offers plenty of tranquility on a Portuguese boating holiday.

'When we first opened, we had a group of retired Belgians who rented two boats," explained Manuel Maia, commercial director of the marina. "They brought a pirate flag with them and one night the pirates boarded the second boat, stole their women and took them off for some wine."

It seemed that I might have been foolish hiring just one boat to explore the Grande Lago, Europe's largest artificial lake. Created in 2002 to provide water for Portugal's sun-baked Alentejo, which covers one-third of the country, the lake is often overlooked by visitors who flock north to Lisbon or south to the beaches of the Algarve. This region is dotted with stunning fortified towns, a legacy of centuries of history as disputed territory, and the searing heat has encouraged locals to cultivate olive trees and vines (the wines from which regularly win awards).

The best ways of exploring a lake is, of course, by boat. The only drawback was that I'd never sailed anything more complicated than a narrowboat along a canal. So I persuaded my car-obsessed husband (who was mildly disappointed that the boat's top speed was 6mph) and two friends to help prevent me causing a shipwreck or – more likely – getting horribly lost on the great expanse of water.

But even for a novice, it's all surprisingly straightforward. The boats come equipped with GPS, sonar and maps; there are some helpful marker buoys on the lake itself; and, best of all, you have the water almost entirely to yourself. Even on the busiest weekend, with all 15 of the boats from Amieira Marina and dozens of fishing boats out, you can go long stretches without seeing anyone else on this 96 square mile expanse of water.

This has distinct advantages. One Spanish woman, concerned what the local authorities might think of her plans to sail in the nude, was reassured by the staff at the marina that no one was likely to see her – and given the laid-back attitude to life in this sun-baked corner of the country, it seemed unlikely anyone would mind much anyway.

The bigger boats on offer sleep up to 12 people, so my group was allocated one of the smaller four-person models, the Campinho – complete with two double cabins, kitchen and a gas barbecue at the back. We powered slowly away from the marina, two rented kayaks safely hitched to the back of the boat, under the watchful eye of our instructor, Victor. A few minutes later, evidently satisfied that we weren't going to sink at the first opportunity, he made his way back to shore and we were off on our own.

This is no ordinary lake. At times it's hard to believe it hasn't been here for centuries, the water shimmering lazily in the sun, reflecting a cloudless blue sky, fields of purple and yellow wildflowers and grasses decorating the hills. Islands are dotted here and there. Trees seem to grow directly out of the lake, their roots lost on a bottom you can no longer see, while roads regularly weave down the hillside and vanish seamlessly into the reservoir.

The islands are in fact the tops of hills, hidden by water that's over 100m deep in places. Created – amid great controversy – by the construction of the Alqueva dam, the lake now fills the valley, and drowned Roman ruins, prehistoric rock art and a village called Luz in the process. Another Luz has been built further up the hill to mimic its flooded predecessor. While it's not an exact replica, planners tried to make sure that old neighbours would again live alongside each other, and kept traditional designs for the low white buildings.

The new Luz still looks oddly unlived in and faintly sterile; more time is evidently needed to knock off a few edges and weather the buildings. More interesting is the Museu da Luz, a modernist structure not far from the dock, which has a great introduction to the lake's creation, and the history of the old village.

Emboldened by a successful few hours of navigation, it was time for something more complicated – mooring up. Thankfully there were only a couple of curious storks watching as we headed towards the bank of a small island, slipped the power into reverse and glided in with a gentle bump.

Leaping off the front of the boat armed with a metal stake and lump hammer, I tried my hand at a few distinctly non-maritime knots: satisfyingly, the rope held. The rest of our small crew fixed the metal ramp to the front of the boat and wandered ashore, then we jumped into the two-man kayaks to investigate the shores at an even slower pace.

Once back on board, we prepared for dinner in our first overnight stop, Estrela. Estrela is a two-horse town. Literally. Both animals stopped grazing to watch as we walked the five minutes from the small quay up the hill into the village. Typical of the area, the whitewashed houses on Estrela's few streets are decorated with beautiful tiles and the simple church dominates the centre. The town also has a fantastic restaurant: Sabores da Estrela, which overlooks the lake and has a patio that's perfect for sunny afternoons.

Here the table was laid with a selection to nibble from – local bread, some fantastic garlicky olives, and a variation on pork scratchings. Indeed, pork featured prominently, from local presunto, similar to prosciutto, to a pork dish served with asparagus and breadcrumbs. It even pops up in one Alentejan dessert: pasteis de toucinho.

While any of the quiet villages round the lake are worth stopping at, don't miss Monsaraz. A beautifully preserved fortified town, it was originally built by the Moors when they controlled this area, before it was conquered in the 12th-century and put under the control of the Knights Templar.

The first view from up the lake is spectacular. As you pass under the new bridge measuring three-quarters of a mile (which required pillars rising 250 feet from the bottom of the valley), you travel from a marvel of modern engineering to somewhere that seems unchanged for centuries.

We moored up at the northern dock, around three miles from the town (transfers can be arranged with the marina, with the alternative being a long dusty trek uphill). I walked through the Porta da Vila gate, one of four into the village, then climbed the precarious steps up on to the ramparts. The view across the olive groves and vineyards to the water stretching off into the distance was amazing.

The steep, cobbled streets twist sharply past the parish church, home to a marble tomb of one of Monsaraz's first mayors, as well as some frescoed pillars, although the decorations below head height have been rubbed away by centuries of people leaning on them. Beyond an array of small shops selling local crafts – particularly traditional painted ceramics and rugs hand-woven in North African style – the castle looms over the town.

Back at the deserted dock after the sunset, I looked up at the blanket of stars scattering the sky, more than I've seen anywhere but the desert. Thankfully, there were no pirates to spirit me away, no Spanish nudists to distract me. It was all mine.

Travel essentials: Alentejo

Getting there

* The main airport for the Alentejo is Lisbon. The writer flew with TAP Portugal (0845 601 0932; flytap.com), which flies from Heathrow and Gatwick. Lisbon is also served from Heathrow by British Airways (0844 493 0787; ba.com); from Gatwick and Luton by easyJet (0905 821 0905; easyJet.com) and from Manchester by Bmibaby (0871 224 0224; bmibaby.com).

* Amieira Marina (020-3026 2551; amieiramarina.com) can arrange car hire with Guerin (00 351 707 27 2007; guerin.pt) from €30 per day. If you book through the marina, you won't be charged for the days you're on the boat.

Sailing there

* In September, three nights' weekend rental of a two-berth boat with Amieira Marina (020-3026 2551; amieiramarina.com) costs from €872; one week costs from €1,341. Transfers to Monsaraz cost €10 and should be pre-booked at the marina.

More information

* Alentejo Tourism: visitalentejo.pt