From May till April? Aren't they closed during the rainy season? I was in the Surins last March...same same but different, more snorkelling orientated, Similans are better for divers I believe. You can get there from Khuraburi...very easy to organize transport to get there without booking ahead though they do get crowded at times...especially Songkran. Mostly Thai visitors.

The NP tents on the Surins were 300-450B/night without bedding though you can rent a (very)thin mat and a pillow as well if you like. Or you can take your own camping gear. There's a cafeteria which has a limited selection of food but it's OK. Showers and toilets were OK too, though there can be queues to use them at certain times of day. The longtail trips organized by the park were only 70B for a half day trip but they didn't actually drop you off at other islands, just the snorkelling spots....don't know if it's the same for the similans.

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.