The FE-606 used either a 50B-6 or a 200B-5S transducer. The FE-700 is an IMO approved sounder that uses either a 50B-6B or 200-8B transducer. The power and impendence specs for the 50B-6 and the 50B-6B are the same so technically you could use the 50B-6. The specs for the 200khz ducers are quite different. Are you trying to use the FE-700 in IMO application? If so, you may need to change the housing that the transducer is currently in.
While both the 50 and 200Khz transducers for the FE-606 had the same shape a gate valve was never mentioned in the latest installation manual. However a quick inspection of the transducers installation should confirm if a gate valve was used. I would be glad to ask if the 200B-5S could be used but before I do can you let me know the flag of the vessel?