Using polystyrene as float/pontoon for a houseboat

The houseboat is going to sit on tidal mud in a river estuary, floating mainly on the highest tides. It will be moored using fixed posts (spud lugs) so that there will be very little lateral movement. The proposal is to build a 2 storey plywood houseboat on a 80cm polystyrene float sheathed at the side to protect it from lateral damage and fire.
Does anyone have any experience of doing this?

 

Expanded polystyrene will initially float on top of the water's surface, but if in permanent contact with water it has a tendency to absorb water very slowly (over a period of weeks) until it becomes waterlogged, at which time it may either float just at the surface or slowly sink.

If you want to use foam you need to use closed cell foam.

On your scale that could be quite expensive, and surprisingly heavy if they are solid blocks.

Best to buy or make hollow floats of plywood / epoxy , or poly urethane closed cell foam ,Polycore honeycomb panels , steel , even concrete modules similar to those used for marina docks etc.