Try these...
Leg extensions
Leg curls
Calf raises
While working out, you'll want to incorporate endurance training with the weights. Use sets with higher reps. As you move from one set to the next increase the weight.
Ex:
Leg extensions
Set 1 20 reps 130lb
Set 2 15 reps 140lb
Set 3 10 reps 150lb
Definitely need cardio!
The above is a nice start. Maybe add some squats, seated squats should be fine. Also, do not foget to work out your core (abs and back). Having a strong core makes everything else seem easy. Cruches, flutter kicks, "superman" pose, lots of different workouts for your core. Get a ball, they are great for core workouts. Pilates and yoga too, believe it or not.
In addition to those exercises, cardio is key. The single best cardio workout for diving IMO is cycling. Outside is best, but even on a stationary bike you can get a great workout.
Try to do interval training, indoors you can get a training video, Chris Carmichael makes some good ones. Outside try to find someplace with some hills.
High RPM sessions help burn out marbalized fat in your muscle tissue as well as work your heart and lungs. Low RPM sessions will build power in your legs.
Running is also good, eliptical machines and a stair climber can help break up the monotony. If you can't run but have access to a treadmill, raise the incline up as high as you can while you walk.
Swimming with fins, really the best way to improve your finning fitness is to...fin.
Of course, if you don't have much of a history with working out, weight training especially, seek out a personal trainer for a session or two, to learn the equipment and the exercises. No use busting your rear day in and day out but doing something wrong, which not only reduces the benefit, but may well cause an injury...
