"they likely would ask for an out of water after seeing the in-water; "
I'd send them a written reply stating that I'd pay for ONE or the other but not both. If they felt that set criteria on the in-water survey would then require an out-of-water, I would want those criteria in writing, up front, so I could determine which survey would be necessary and only pay for the one correct survey. Diplomatically, of course.
Maybe suggest that you could have a USCGAux Courtesy Inspection at no charge, and if the insurer presented a list of written criteria, have those inspected at the same time, so there were objective criteria to determine what private survey would be needed and eliminate duplicate costs.
Of course the GCAux courtesy inspection is a good idea in any case, since they give you one of those "Go away, this guy isn't worth bothering" stickers if you pass. :-)