Building a pontoon dive boat

[ScubaDo]

Older pontoons tend be too small, four divers with gear is about max weight.  You also can't cruize with any speed when weighted down.  I modified an old aluminum ladder and mounted on side of boat, using the pontoon as support.  Must be able to hinge out of the water.  A big motor will be wasteful, adds weight and can't cruize fast anyhow.
If you can add a third pontoon, do it.