I think that situation may have changed this year, according to some posts on this thread [Thailand in August - Andaman or Gulf? (or: Is Koh Tao really that bad?)] It would appear that KT is now not affected by the general summer downturn in Thai tourism.
These comments actually persuaded me to change my mind on my planned dive trip for next week, and have actually now decided against KT and made arrangements to visit Koh Lanta.
July / August has always been the peak season for the Gulf of Thailand, since the conditions are usually better than during the Northeast Monsoon.
Regarding Koh Lanta I would suggest to check with the dive operator you would like to dive, since most of them do not reopen until about October. Other destinations in Krabi (e.g. Ao Nang, Phi Phi, Railay Beach) or the quieter areas of Phuket, may be a better bet if you actually plan to go diving.