I live in Hawaii and we board in salt water only. We bought a new boat off the lot which already had a shower, bow filler cushion, and teak platform on/in it.
1. Shower: I added a 40 gallon ballast/shower supply bag in the bow of our boat. I ran a hose from that bag to the inlet of the shower pump. We fill the bag with fresh water. This gives us a fresh water shower. Last week I added about 70' of hose in large coils inside the engine compartment (v-drive) between the outlet of the shower pump and the shower head. This gives us about one minute of hot water after a 15 minute set behind the boat. This comes in handy when the weather is cool. My wife loves it!
2. Bow filler cushions: We have them. Some of our friends prefer lounging/riding in the bow so the filler cushions make it nice.
3. Teak vs. fiberglass: I have no experience with fiberglass. I really like the looks and function of our Teak swim step. A little work to maintain but not too bad.
4. Floor mats: I don't have them.
5. I recommend PP. It allows the driver to pay more attention to the rider, the boat indications (temp, oil press, etc), other boaters, and obstacles in the water.
6. I agree with WakeNup on the galvanized trailer and the transom flush kit.