Bobad,
Relax, the 2x12s are just fine, think about how many times they will be exposed to underwater conditions vs dry, you really did good.
You might take a look at a marina if there is one near you for some ramp ideas, the salt water guys deal with a couple of tide changes daily. The one style that comes to mind is a set of rollers on the wharf end, use iron plate for the rollers to glide along on.
