So I've been using the microphones the last couple days and I'm pretty happy with them, barring some minor issues.
I wish I'd thought of the fact that the hydrophone needs to sink. There ought to be a washer or something inside the second layer of hot glue.
Because I'm not using a preamp the gain needs to be cranked when plugged into either the external mic or line in inputs, so there's some white noise. Guess that's something else I'll be building soon...
Here's some stuff!