I don't know about your area, but in ours we just had a haul-out/spash to replace the prop for $180 ($6/ft). If we had a leak, never mind one so bad we ended-up with standing water in the cabin, that boat would be hauled at my soonest opportunity. I'd take a vacation day to get it done. I think you're being way too cavalier about this.
As far as just yanking the knotmeter transducer: 1. A GPS is not a substitute for a functioning knotmeter, IMO. Sure, a GPS will tell you your SOG, but it can't
tell you your speed through the water. 2. A GPS is a wonderful thing, but it's a compex piece of electronics that's dependent on the good graces of others. We rely on ours. But we also have a knotmeter, compass and charts. 3. I bet it will be more difficult to properly patch that hole in your hull than it will be to repair or replace that knotmeter thru-hull.
If you do choose to replace the thru-hull: Do your homework before
having the boat hauled. It's probably specific to your knotmeter transducer. You may have to order it somewhere.
I doubt you'll find a sealant that's going to fix your problem on the cheap. But I've been wrong before. Even if I am: This is definitely
not the way I'd
go. But we love our boat, so we want her to stay afloat and be happy