The JSN-SR04T don't work underwater and it is not the same frequency compare to the lucky fishfinder so we cannot switch the transducers.
If your project can work with a open case lucky finder, I can help you make it work by guiding you cutting specific trace on the pcb and then tapping the signal to an arduino. It work very well and I was able to improve the short distance sensitivity from 0.9m to less (value somewhere in previous posts) since I d'ont care about fish finding or echo shape. Unfortunately a open gut lucky finder don't suit my need. So this is still a standby project in my case. I ordered some transducers but have'nt find time to test them yet.
For this to work, you need a Mosfet like this :FQP30N06L or hack the onboard triggering transistor array. Both work and the latter might be easier.