Here's a piece I wrote for my "Ask RSD" column "Rodales' Scuba Diving" a while back. I have edited it & added a few websites to fit your question.
"How to Find a Buddy
by Doc Vikingo
Because when you've paid good money to dive, you don't
want to be stuck with the last loser standing.
Unlike the animals on Noah's Ark, dive travelers do not always board the boat two-by-two. In fact, many divers travel alone or travel with non-divers and depend on their good looks, their charm or the luck of the draw to find a dive buddy. If you're planning a trip and you're not bringing your own buddy, what can you do to increase your chances of finding a buddy who will be a good match for you?
Do Your Homework There are several options for finding a dive buddy before your vacation. While advanced planning isn't necessary, it does offer the opportunity to find out something about the experience, skills and personality of a possible partner.
Hit The Net. You can both locate & post an interest in securing a dive buddy for specified dates and locations on dedicated international or national sites. Some general scuba message boards have a section dedicated to divers searching for buddies, such as Scubaduba's Dive Buddy Directory at
http://www.scubaduba.com/schome.html. Here's a local Tampa buddy board,
http://www.divebuddy.com/Florida-T-Z.html and here's some local Tampa shops & charters
http://www.florida-scuba.com/tampa.htm
You may also post an inquiry to one of the general travel boards that focus on the particular destination you'll be traveling to, Message Board, although these often do not have the specificity or reader traffic of broader-based scuba boards.
Call Ahead. Another option is to contact an area dive club to find out if it has any diving planned during your stay or can otherwise assist you. There are clubs around the world that welcome visitors and readily include them in diving activities. For listings of dive clubs and contact points, see Scuba Yellow Pages
http://www.scubayellowpages.com/ .
Finding a Buddy on the Boat It's not necessary to have a partner lined up before your dive trip. In fact, it's common to find other divers on a dive boat who are also without a buddy. Dive operators will make sure you're paired with someone, and may buddy you up with a divemaster in the event there are no other unbuddied divers. But if you want to increase your chances of finding a buddy you can live with:
Act Fast. As soon as you board the boat, start looking around for someone who might be a suitable match. You can usually tell which divers are without a buddy when they board the boat. And you can also usually tell by the equipment they're carrying whether they'll be shooting photos or shooting fish. Try to find a buddy whose objectives for the dive are similar to yours so you're not at cross-purposes.
Talk It Out. Strike up a conversation to find out even more about potential buddies. By doing the work yourself, you won't have to wait for the divemaster to match you up with someone who may or may not be a good buddy for you.
Go or No-Go? OK, so you've either chosen someone or been chosen by someone as a potential buddy. Now comes the moment of truthyou've got to decide whether you really want to buddy up with this other diver. If you are not willing to take on the buddy responsibility with this particular diver, you must say so, tactfully but clearly. If you let another diver enter the water in the mistaken belief that you are going to be his buddy, you are morally and perhaps legally responsible for the consequences."
Hope this was helpful.
DocVikingo