Not a problem at all. I understand that in the Caribbean, it's customary to just chuck pairs of divers into the water, but in SEA, all dives are guided, so you're a part of a group of up to four divers.
I've met a diver there that was fresh out of her AOWD course, she came out of the trip very happy.
The home base is in Ranong, but last season they were cruising up and down the coast - Ranong to Khao Lak, Khao Lak to Phuket (not Patong though; they docked in Phuket Town which is on the other side of the island), then back up the coast to Khao Lak and Ranong again.
I haven't used it myself, but from what I saw, it's all in good condition. Haven't seen other divers have any issue with it.
It's a mix, served buffet style. The couple that runs the boat is French, but the chef is Thai.
Luck of the draw, really. Early and late in the season, chances of rough weather are higher, mid-season like February it should be calm.
May would be pushing it with regard to weather, but anything up to March, possibly April is good.
If you're comfortable on a scooter, get one in Phuket; the taxis there are infamous for their pricing. I wouldn't ride one in Bangkok traffic though.
Don't think there's any specific time that's better than others; don't count on seeing one - they're getting pretty rare these days. If you want guaranteed whale sharks, there's Oslob in Philippines.
Group sizes, accommodation (en-suite showers, aircon, etc), specific routes.
It's certainly a good investment. When you're diving four times a day, hitting the NDL gets quite easy.
