I've spent a lot of time contemplating something similar as well. The problem with the mirage drive in this application is that the pedaling motion is reciprocal rather than circular, so it doesn't couple mechanically with something like a BBSXX style mid-drive in a straightforward manner. At least not as easily as the bicycle chain driven human powered boats that are out there which are turning a propeller.
Have a look at the many links from the openwaterbike project page if you haven't already
The most common of these drives is either a twisted chain that reorients the axis of rotation 90 degrees from the crank to the propeller under the boat, or a long flexible drive shaft with a right angle gearbox coming off the side. Both of these would be very straightforward to add assist with a bottom bracket mid-drive since they're using rotating cranks.
Normally the total gear ratio is on the order of 6:1, with the propellers spinning underwater at around 500-600 rpm, and that's within the realm of what you can do with a single stage gear transmission from the pedals to the propeller shaft.
There are sine giid discussion of this topic on the various boat forums and HPV forums, but I'd like to see more electric assisted watercraft talk here on ES too. It's an area that hasn't been explored nearly as much as it should be since there's a lot of potential overlap with with ebike gear.
Currently recovering from the Suntrip race on a back to back tandem solar powered row/cycle trike
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Now back in Vancouver with my Big Dummy Frame (yes This One
, thanks ES!) with Grin all-axle front hub, Phaserunner controller, and 52V 19Ah Cellman triangle pack
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