Definitely go with the high density styrofoam. If water gets in the boat the pink stuff will absorb the water and become waterlogged. Even though the HD can get water logged if left under water. example leaving plug in while boat is sitting outside in a rain storm. The HD will release water and dry out faster.
If you remember back in the day, most floating boat houses, duck blinds had the HD foam under them. Even the some metal pontoons where filled with the HD foam.
Carpet: I agree, Lowes and Home Depot has a great selection of outdoor carpet. I prefer the Gray color due to the fact that when it fades it still looks good vs a dark color like blue or reds.
Just my thoughts