These are very small sections. You could just drill holes in the deck on about a one inch grid, apply a heat source like an electric
heater underneath and dry out the core. Once dry, inject epoxy
into the holes and Bob's your uncle. I'm assuming that your core is rot
proof, otherwise not balsa or plywood
, and the frp deck laminate has just detached from the core.
I would also rout out the core around the fasteners for the pulpit and any other hardware
in the area. Use a Dremel 199 bit in a Dremel tool to rout out the core then fill with epoxy
thickened with West Systems 404 or other non compressible filler. This will keep the fasteners from compressing the core and seal out any possibility of water