The sandy shores of Sri Lanka's southwest or those in the vicinity of Galle are best visited in mid-December through to the end of February. This includes Hikkaduwa, Unawatuna, Mirissa and even Tangalle. And while this area is also decent from mid-July through to mid-September in terms of the relatively low rainfall, the sea can be fairly rough, limiting your ability to partake in water activities. The beaches are lovely outside these months, but occasional winds contribute to rough and choppy seas.
The east coast, including the popular surf spot, Arugam Bay, is nice from June to mid-July and then very appealing until late September. This area is still quite nice from October through to mid-December, but there can be sustained rain. From mid-December to February, it is not advisable to visit this area for beaches.
The northeast coast, including Trincomalee and Batticaloa, has experienced some development, and there are some lovely places to stay. It is, however, sparsely populated and could use some trees.
Mirissa is a popular surfing spot and a calmer area than Tangalle, which has quite rough surf. There are various operators offering surfing lessons, and there are many boards for rent. Mirissa has a collection of properties on offer, mostly along the beachfront or slightly back from it. A couple of properties are trying to position themselves at the high end of the price/service/comfort spectrum, but the bulk of accommodation here is cheap and cheerful and is of excellent value for much of the year. Mirissa is a great place to learn how to surf if that takes your fancy.
For a weeklong adventure of Sri Lanka that includes a visit to Mirissa, read this article.
The geography here makes for some lovely coastal scenes: rocky cliffs and outcrops with crashing surf. The resorts in this area are either large-scale, a tad institutional, or budget-level wooden bungalows. With that said, new places are opening all the time, so the possibilities continue to grow.
Read Ancient History & Pristine Beaches for a Tangalle-featured itinerary.
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Probably the best option if you want premium accommodation, picturesque beaches, and accessibility to Galle–it's only a 15-minute drive to Galle Fort. In addition to the larger resort properties, there is a good range of villa properties available for medium to extended stays. There are also a couple of fantastic beachfront restaurants in this area.
Check out this article for a 10-day itinerary that includes time spent on Unawatuna.
One of the longest established tourist beach areas in Sri Lanka, Hikkaduwa offers plenty of activity, good nightlife, and some laid-back, tastefully designed accommodation. The beach is a little tatty these days, with lots of restaurants, bars, and guesthouses along the shore and the busy Colombo-Galle Road ripping through the middle. It is, however, an excellent place to surf and kitesurf in March, and there is a reef just offshore, so you can go snorkeling, and your chances of seeing turtles are fair to medium.
Arugam Bay in the southeast of the island has an exciting vibe. It is undoubtedly a surfing hangout, and hence experiences pronounced seasonality in visitor volumes. Accommodation in Arugam is almost entirely at the budget end of the spectrum. If you don't surf, there are better beaches, but if the seasons match and you are in the neighborhood, maybe wanting to visit Gal Oya National Park, it's a fine place to stay. There is some nice riding/walking/trail running in the hinterland nearby, although there is a bit of an Elephant hazard at times.
Batticaloa & Trincomalee
Trinco is probably the most appealing of the two locations and is considered the Northeast. These are primarily Tamil areas, making them culturally quite different from the southern beaches, which are all Sinhalese areas. The beach is lovely and, most importantly, is largely bereft of beach bars, crowds, and hawkers. There are a handful of decent resorts, and Trinco is only two hours or so from the Cultural Triangle area. The motivators for visiting these locations are likely to include the weather and proximity to the Cultural Triangle if you are aiming for a compact trip.