A guidebook would be good way of getting some introductory knowledge. Depends on what style and taste of travel. Traditional style budget travellers who liked to stay in simple wooden bungalows are mostly appalled at Krabi and Phuket. Beaches over developed and expensive. Railay Beach East and West are particularly expensive and for some tastes too built up . Ao Nang is a bit like that too. Tonsai beach is rocky Krabi is a coastal area not an island.
The main beaches of Phuket although once paradise like are heavily built up and Phuket is one of the most expensive provinces in Thailand. But if you like developed resort locations and want to stay in hotels maybe you might enjoy them . Unless you go on boat trips to offshore islands the beaches of Phuket and Krabi not that special for snorkelling. Patong beach , Phuket once had excellent coral reef and clear water decades ago but not sure what its like now.

PICTURE - Phi Phi island before mass tourism when mainly budget travellers stayed there and it was possible to find bungalows for 50 baht.

I think she meant she is leaving Bangkok to go to the islands at the end of March. Point taken about cyclists keeping on the move although nowadays on the islands rather than backpackers making a place a destination its increasingly mainstream tourists taking over from the backpackers . Ko Phangan is case in point.

Rock climbing tends to be done in Krabi , May is start of wet season there so water maybe not that clear for diving and for climbing it could vary from day to day. Otherwise the weather at that time is okay at Ko Phangan , you could try rock climbing there as well dive courses.

When are you going ? not so many islands with coral right next to the beach.

We are looking for snorkelling. My idea would be to complement snorkelling tours with some snorkelling near the beach, close to the hostel ( like in koh tao or Gili islands, but not Koh tao we have already been there).

Thai islands don't have many hostels on beaches . Budget accommodation usually bungalows.

Budget accommodation in Thailand tends to take the form of guesthouses rather than hostels although there is a blurring of the two sometimes. Having said that the number of hostels in Bangkok and Chiang Mai has grown but there are more guesthouses. On the islands most beaches don't have hostels - tends to be resorts or bungalows . Phuket however doesn't really have any bungalows anymore and is expensive and some beaches do have hostels.

See also these threads for more info about the above -
Ko Phangan Report

Kapas Island , Malaysia Thread

Tioman Island , Malaysia Thread
Sumatra Update

The idea that there is a mass of under 40 people who are enamored by the full moon party and who its impossible to have conversations with doesn't reflect the reality. Even in the FMP's wild heyday the great majority went to Ko Phangan for the beaches rather than party scene . And these days the FMP is a commercialized tourist attraction anyway you could go to it even if you were 75 years old.
The clientele is something you have to take chances with , as you say beaches are not all split up into age groups.