I need more ideas on underwater transmitter and receiver circuit designs

Hi Bukola, i think you can use photon detector circuit interfaced with high precision optical transmitter to achieve the task.But which type of data you do want to send in your project?

A few year back, I was on a project of long range communication, where the communication setup made is of underwater type, for that purpose we team members perform an activity for testing using optical fiber cables of different types i.e., LC-LC, FC-FC, LC-FC, SC-SC etc. and some laser source at two ends. This technique is used in telecommunication site work where networks communications between countries made underwater using fiber. Here I would like to recommend you to visit SSLA for getting more assistance in this project.