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I hear you on drilling holes. I did have to drill 3 holes in the transom to mount my transducer. But after having issues, I am filling those and using a block similar to the Stern Saver where you glue/stick the block to the outside of the transom, then you are only drilling into the block.

You could secure a bilge pump with 3M tape. The only downside is, it's now higher off the deck equal to the thickness of the tape. I guess that's not a big deal. Let me know how it goes so I can do it too :D