How to transmit data underwater?

The circuit looks OK to me, although I haven't checked all of your component values. However, you haven't specified the value of the inductor in the transmitter, and the capacitor value of 0.158uF looks rather high. I would go for something like 160uH and 3.9nF, which resonates at 201.5KHz.

You will need to get the high and low frequencies the right way round for the async signal to be the right polarity for the FTDI chip.

10M is a rather large value for R13. I think you may need a smaller value for better noise immunity, in the region of 200K to 1M.