I was thinking about Ceasarstone in Blizzard. Any thoughts?http://www.caesarstoneus.com/products/color/2141/

You can use side panels or a deck/surround to create what is, in effect, a free standing bath that would work well aesthetically and, optionally, as part of a wetroom-style bathroom. Deep soaking tubs are especially effective for such projects; they have smaller footprint, which affords more room in the shower area, but their greater depth makes them exceptionally comfortable.

I think rather than fixing any seating - keep it flexible - as despite being a floating home - it looks to me to me moored in one fixed location ? So if you keep the seating flexible you can follow the sun (or shade ) ... 2x three seater or four seater sofas could potentially double up as two sun loungers - might be worth looking at... are you thinking of an outdoor kitchen up there too ?

If you are having wall mounted taps, then the wall has to be chased out to allow for the pipework, so, as you point out nothing will be on view or accessible. Thus regardless of whether or not you then tile, and you had a problem with the plumbing ( maybe not the tap itself ), you would have to dig everything back out. We had Grohe Wall mounted taps and have just changed them for Burlington as we are upgrading the bathroom. We never had a problem. It's a risk you take.