I recently upgraded to a combo fish finder GPS unit. I was using a suction cup to hold the transducer to the side of the W fishing kayak but I noticed it would create drag and occasionally fall off. I tried several different methods to mount it in the inside of the W and shoot it through the hull, not so successfully. Again sometimes the simplest method works the best. I wish I would have tried this first. On YouTube tube I noticed a installation using duct seal. I purchased 1lb. at under $3.
Simply form a well at the bottom, lay transducer flat to surface , pinch some of the duct seal over it to hold in place. When you start kayaking simply put a slight amount of water in the well you formed to cover bottom of the transducer and start enjoying drag free readings. This duct seal sticks great, is waterproof, remains soft, and is portable. If you want to move it to another W simply peel off and reset it.