https://www.futurefoams.com.au/marine-buoyancy-foam/

there are others 

i would shy away from a expanding foam - asking for issues with corrssion as water gets trapped 

cut and shape such that u can still  rinse the inside of the cavity

 pascoe industries if they still exist makesa marine grade foam -as a 2 part 

i have used it but u needed to calcuatevery exactly the internal size as once u mix and pour itwill bend decks out of shape if not enough space to expand

i made a poly craft 4.1 look like a pregnant balloon when i miscalculated

 if you have a sealed deck then extremely hard ,and not sure ur going to achieve your intended result ,would not use expanda foam , i have used foam sheets cut to whatever sizes and shapes it need to be   on my old plate ali boat  after removing the plate on top of the petrol tank , also cut inspection hole in the kill tank & forward deck ,then pop riveted back and re sealed with  marine grade silicone. be sure you go through all scenarios before you start, or you will regret starting