I'll just add to the above that it helps when descending to use a reference line if you can. You said you have some buoyancy troubles so the ability to stop your descent (for plugged ears, for example) is just as important as getting it going so descending along a rope or anchor line is important in deep water.
Also, resist the temptation to add weight to sink. If your weight is good (or good enough--let's be pragmatic) then you will be better off in the long run to work on your technique. A good thing to do is to go a day before the boat dives and do a refresher with a local professional. They can help you sort out the descending thing and check your weight with you. Don already suggested that but I would like to add my voice to this suggestion.
Good luck and have fun!
R..
Also, resist the temptation to add weight to sink. If your weight is good (or good enough--let's be pragmatic) then you will be better off in the long run to work on your technique. A good thing to do is to go a day before the boat dives and do a refresher with a local professional. They can help you sort out the descending thing and check your weight with you. Don already suggested that but I would like to add my voice to this suggestion.
Good luck and have fun!
R..