Well one thing you can do is to use small transducers to ping and range gate on the ping.
You would have to have a detector circuit which could be very simple to make.
Here is a link to a transducer that could be used: http://www.newark.com/prowave/400ep250/transducer-ultrasonic-tx-rx-40khz/dp/98K9162?CMP=KNC-GUSA-GEN-SHOPPING-NEW-PROWAVE&ad=81515566821&gclid=CMHl-7yT4swCFdRZhgodUJwNiQ
What you would need to do, because of the voltage to drive all of them, is maybe make a switching circuit and you rotate between transducer heads. You ping / listen to calculate if an object is within your ten foot standoff distance and act upon that if it is the case. Considering that sound travels on average around 4850 feet per second in saltwater (or a little slower in freshwater), the amount of time is going to be REALLY small. This would give you plenty of time to cycle between all of your sensors. You are looking at a couple of milliseconds per sensor to create your standoff zone.
One of the things you are going to have to do is to seal off the area where the electrical connection is and create a mount for them.
Well that is my initial two cents on the situation.