Backpacking with a heavy bag is a real PITA and costs money, as you end up taking more cabs or staying in more expensive places just because they are closer to the bus station or pier…
My first trip in Mexico/Central America, I ended up giving away half of my stuff after a few weeks as I was so fed up with carrying it around (I wasn't diving back then, but had a mosquito net, 5 changes of clothes, 4 pairs of shoes, and way too much "just in case" medicine).
I did a bit of research about travelling in Thailand (haven't been there yet though), it seems you can find most things you need there. No need to carry six months worth of shampoo or soap, bring sample-size to get by for a week and buy more as you need.
Clothes can be washed : a couple T-shirt, a long sleeve shirt, a pair of long pants, a pair of shorts, 2 bathing suits, a fleece jacket, maybe a light raincoat or umbrella, 1 pair of walking shoes, 1 pair of beach sandals, a hat, sunglasses, a few underwear and socks and you're set.
Soak your clothes in a special DEET-based product that you can buy in outdoors stores, it helps keep mosquitoes away.
Microfiber towels are great for travelling, they're light, don't take up any space and dry superfast. Sarongs make great beach towels and can be used as a sheet too.
Other useful items include a flashlight, padlock, maybe a silk sleeping bag (lightweight, small, supercomfy) if you're gonna be staying in cheap places and are weary about the cleanliness of the sheets.
Do you absolutely need to bring your laptop ?
There are Internet café on the touristy islands. And if there aren't, it's unlikely you'll be able to get a connexion in your hotel room either. As a matter of fact, if staying in the cheapest digs, you may not even have access to an electric outlet to charge the battery.
If you just want to download your pics, you could invest in a portable hard drive, download the pics on the Internet café's computer and copy them to your USB drive. That's what I've been doing these past few trips in Mexico.
Since you're gonna be staying so long, ask about the possibility to buy a cheap local cellphone and SIM card. I don't know if it's an option for tourists there, but would be more useful and cheaper than making local calls using your Belgian phone thru international.
As far as the bag itself goes, forget the wheeled kind, unless you plan on doing most of your travelling by plane and stopping in less than different places. Wheels are heavy, take up space, and are useless on sandy or rocky roads.
My preference would go towards a backpack big enough to hold your wetsuit/BCD/fins/clothes/toilettries/sandals that you carry on the back, and a smaller, cabin-sized one, for the reg/dive computer/camera/housing/important medicine/other valuables that you carry in front of you.
Big amounts of money, passport, credit card, plane ticket go in a money belt under your clothes.
Here's a great Website about packing :
Leisure and Business Travel Packing List - Travel Light with One Bag!
