I'll play the devil's advocate for you here. This is NOT a criticism. It's something you MAY or may not have considered and could become your worse nightmare. First of all, we'll have to assume that every (most) major players will license you to use a third party accessory on their product without voiding the waranty. If that is taken care of, great. Now, we have a customer who buys a unit (any manufacturer) let's take Humminbird as an example. The customer loses the bottom at 20 clicks. The customer contacts Humminbird. Which questions will Humminbird ask the customer (for example)? They'll want to know which transducer the customer is using. Then the fun begins....is it the unit itself or the transducer that's faulty? Is the transducer properly installed? With newbies it often is not properly installed.
Does the customer really want to deal with TWO different suppliers for one item on their boat? I don't know. How have you got that covered so far?