I finished rigging the life lines a while back, used the boat for diving, and thought I'd share the results. The biggest challenge after learning how to splice rope was to rig the ropes in such a way that when tension is put on them the D ring points straight down as opposed to being pulled at an awkward angle thus making it more probable that it will eventually tear out. I had to redo the splicing a couple of times to achieve this. I can't say I have a formula to get this right, I just had to go by feel.
You can see in the pic how far down the rope sags when pulled. This had no bearing for my purposes as to get in the boat we'd grab the rope by the thick spliced part near the D ring. The sagging is lessened because the rope doesn't run free through all the D rings, there are two sections per side therefore 4 attachment points per side. You can also see the rope free section which is the boat exit/entry point, visible is the chaffing that occurred when using that entry point sliding over the rope that was there originally.
I'm pleased with the results and would recommend rigging this way for a similar application. Thanks again everyone for your suggestions.