OK, I believe it is possible to buy prescription masks for short sighted people, but can these be bought easily from Thailand? Or, preferably, is it common for dive shops or resorts to loan them out?

If buying / borrowing, how much is it likely to cost comparted to a normal mask, roughly? And would I have to order in advance to ensure of the correct prescription to match my eyes, or do they generally have large stocks of many different prescriptions, and so getting one there and then in the shop wouldn't be a problem?

Any help on this would be greatly appreciated. Thanks.

It is better for you to purchase your own mask with prescription lenses. Most dive centers do not find it economically viable to carry an entire range of masks with the various prescirptive lenses. For example a Tusa Splendive mask retails here on Phuket around 2490 baht and the lenses each at 1050 baht. Many dive shops have the basic prescription lenses. So if you need a really specisl prescription sort it out before coming to Thailand.

Thanks for this Chantal, I'll keep it in mind.