Like the OP, I try to find creative (home made) solutions to problems. I agree this is impractical, but I also feel like we've been a bit harsh on the OP. I hope he doesn't take it personally. (or maybe I'm just feeling overly sensitive today?)
Some of the posts have the fluid dynamics incorrect. We use a 10 HP pump to pump water up about 150' in a 2" pipe. This equates to about 66 PSI or about what a strong tap water supply is. You don't need anywhere near this level of power to move the bow of a 25' boat. I haven't done the calculations, but imagine what a 10 HP kicker would do? The problem with a sump pump is that it moves large quantities of water at low velocities. To make a pump powered system work, you need a high velocity pressure. (essentially the mass x velocity of the water at the exit point. As opposed to the static pressure which is what we usually think of when we say "pressure") So the interals of the pump are what makes it succeed or fail. Just like prop selection, we need to find the correct volume and velocity and impeller to create them and then determine how much horsepower it would take to make that happen. Plus any loss through the piping and fittings. I found a pump capable of moving 60 GPM at 60 feet TDH. Roughly 26 PSI. Probably good enough to move the bow for what the OP expects. Takes about 2 HP. The pump I found costs around $1500 in 220 VAC format. Probably close to $2000 as a DC system and not sure what voltage but at least 36 , maybe 48. Now you need 4 more batteries and probably an additional alternator on the engine, which would be a custom job. Pipe, fittings, hull fittings, wire, fuses, switches, valves. Suddenly, the costs do indeed exceed the bolt on trolling motor type or a regular hull mounted thruster.