Considering the construction principal: "Water Will Find The Failure," water-proofing barriers must be complete, or provide outlets.
If you seal your object 100%, it's ability to retain water will increase as water will be trapped by both the foam and the sealant. However, if the foam contains 0% humidity when you seal it and it is sealed 100%, then it will be perpetually dry. In both cases you are Maintaining the buoyancy of the object.
If you fail to seal the object 100%, or it suffers damage or degradation, then the object will have a tendency to retain water as it is added to the system and will generally Lose buoyancy over time with exposure to moisture.
Most foam floatation I've seen is protected from UV behind wood or fiberglass bulkheads that contain a drain to the bilge or cockpit. Through either damage or degradation the waterproofing fails and water will find the failure.
If the foam is holding water, it is more indicative of aging foam that should be replaced, not preserved.
Best of luck with the boat!