We bought a 20L dry bag when we arrived in Krabi, and it was a life saver! It only cost us 700 Baht (approx 20 USD) and it fits both of our laptops with plenty of room to spare. It has a handy strap as well making it easy to transport around. I wish we had purchased it earlier — it gave us comfort against water, sand, and just general safety against the elements.  Definitely worth it – you can find a range of bag sizes all around Thailand – but be a bit discerning as prices vary from place to place. We bought ours at an official shop and it was about 100 Baht more than can be found in the markets. We figured it was worth the ease of mind to pay the extra 100 baht.