i like beanbags for seating, somebody will groan, I say that all that time.
Comfy and flexible. take only what you need. Of course you need some place to stash them when you get to diving, I have a t-top and a slightly larger boat. Still you could probably work something out on an A-frame. Same for speargun racks, best thing ever. Easy to whip up with sratboard or even PVC pipe. Again, mount to the a-frame or maybe even the outside of the transom.