Buoyancy Foam >488m

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@Tides Have you seen this post?

Epoxy with Fiberglass cloth for reinforcement for Foam flotation

Hi Jim and Eliot: Here at Mission Robotics we’ve switched to epoxy-coated floats on all of our BlueROVs. The original intent was to allow deeper diving, as the Last-a-Foam 3318 is only rated to 250m in its uncoated form. We do a lot of testing in Lake Tahoe, which is 500m deep, and wanted to be able to routinely operate in any portion of the lake. After we started playing with epoxy-coated foam, a number of other advantages became apparent. First, by skipping the plastic fairing and attachi…

Something like this foam might be worth considering since it’s made for the depths that you want.
https://esyntactic.com/products-solutions/syntactic-buoyancy-materials/microsphere-syntactic-foam/