: A word about the polyurethane glues, such as Gorilla Glue . . .
: they aren't getting good reviews for marine use,

Au contraire, they work very well, but they REQUIRE very well fit joints and tight clamping. Do that and you will have no problems.

: and they, also,
: require close fits and good clamping - they are definitely not
: gap filling, even though they foam up and that foam fills gaps
: (foam is mostly air . . . .).

Absolutely true, they should never be used in poorly fitted joints or for gap filling. They actually will fill gaps...with useless foam. It's roughly equivalent to trying to glue things with spray insulating foam. You can imagine how (in)effective that would be.