@JimInPB: I think you're right: "you may not be so bad off with what you have" even when foam gains somew weight from water. Even soggy coffee-cup foam blocks take a long time to get so waterlogged that it no longer provides helpful buoyancy. But to stay safe, you have to know how to calculate how much buoyancy the foam is likely to lose over a longer period of time. You can reduce risk by installing enough volume of foam to provide a safety margin and by checking periodically to ensure it's fairly dry.
The procedure is, in theory, straight forward: Experiment with the foam to discover how much water weight it will absorb per cubic foot in any given time. For example, if a cubic foot of foam absorbed 3 pounds of water in a month, it lost 3 pounds of buoyancy. But it's hard to find real world, experimental data.
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I found ONE academic research paper on water absorption rates for expanded polystyrene aka EPS (coffee cup style beaded polystyene). It's got some real data about how much water EPS soaks up over time.
You can read it at the following link: http://leidykla.vgtu.lt/conferences/MBM_2007/1pdf/Gnip_Kersulis.pdf
[Disclaimer: It's been almost 30 years since I read and wrote statistically articles similar to this. I'm rusty. I had to read it several times just to get the gist of it.]
The authors collected water absorption rates for 162 samples of EPS of varying densities, from various European manufacturers/ They took measurements after 7 days, 14 days, etc, up to 250 days, They recorded their observations in terms of change in mass of the sample and converted that to % volume that was saturated.
At the end of 28 days of submersion, the average quality of foam of one cubic foot of EPS took on 1.92 pounds of water and lost 1.92 pounds of buoyancy.
At the end of 28 days, the worst performing foam sample group took on 3.12 pounds of water per cubic foot and lost 3.12 pounds of buoyancy.
Then the authors tested the rate of water ingress for longer periods. They observed that the foam continued to soak up additional water over time, but at an ever decreasing rate.
At the end of 250 days, the loss of buoyancy and weight gain was only 1.6 times greater than the 28 day figure. The worst performing cubic foot of foam gained about 5 pounds of water and lost 5 pounds of buoyancy.
Then they derived some equations from all the data to predict how much water the foam would soak up over even longer periods of them.
Their predictions:
At the end of 2 years, the equations in the article predict that each cubic foot worst performing foam will have absorbed 7.08 pounds of water and lost the same amount of buoyancy.
At the end of 5 years, the equations in the article predict that each cubic foot worst performing foam will have absorbed 15.33 pounds of water and lost the same amount of buoyancy.
At the end of 25 years, the equations in the article predict that each cubic foot of worst performing foam will have absorbed 21.6 pounds of water and lost the same amount of buoyancy.
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