There are a bunch of consumer-grade devices that will convert line/speaker jacks to bluetooth.
As a random example, here's a battery-powered box that sends/receives headphone-level signals to bluetooth and back the other way.
Downsides, it's another thing to be carried, and another battery to be charged.
This example image doesn't look waterproof given the open jacks, but the whole thing could sit in a sealed plastic bag/box and even be secured to the boat in case of rollover.
Your radio would have to have a headphone socket, and probably needs a VOX transmit mode so that you can speak without having to press a button.
Upside - a separate adapter like this could be used on other devices that lack bluetooth. A home stereo, a television, anything that has an audio out jack. Even a retro computer could send via this to your bluetooth hearing aid without needing to support bluetooth directly.
If your rowing club/team already provides compatible radios then owning something like this could be a net saving.
Side thought - If your rowing team never transmits, it may be more effective to choose something like this that supports peering to multiple headsets at once. That way all rowers could listen to one radio in the boat.
Cheap BT earpieces are much more "disposable" than expensive radios per-person.
