What size engine would it take to tow 2.5k (boat)?

I wouldn't do it with a Subaru.. not all about towing.. stopping is just as important.

Walt brings up an interesting point, as usual. I've sold hundreds of 2.5i Subarus that have performed exemplary service towing up to 2-3k lbs. The strong torque of the gutsy 2.5i and older 4EAT autobox is dependable too. The results aren't in yet for the softer-accelerating CVT matchup. (Certainly I would NOT recommend the wimpy 2.0i (Imp/CrossTrek) for towing anything!) CVTs are failing both because of electro-mechanical problems with their throttle bodies (which may NOT be affected by load), as well by wear of internal bearings (which WOULD be affected by load). So we're all CVT guinea pigs here. Back to Walt's comment: Subaru's chosen brake rotor sizes have always been on the edge of acceptability for both size and corrosion resistance. Inadequate FRONT brake performance is usually easy for a driver to assess in casual tests. What's less obvious is that as the hybrid REAR rotors (which are undersized as they share double-duty as e-brakes) become compromised they significantly drain system pressure, thus extending braking distance. This is ironically especially noted when braking just before a stop: "greasiness" in the last few yards is usually about REAR compromise. So make sure your brakes are up to towing a heavier load, think twice about towing with a CVT, and don't think for a moment that getting a 3.6i changes any of the fore-mentioned concerns!

change "fore" to "afore"; losing it in my senior years....

Towing a 4x8 trailer with a 42" riding mower was fine (even at 80ish) but I wasn't impressed with slowing down. Could be not-so-firm tires and the rear brake hybrid deal you speak of..

Yes, ya do want H or V-rated stuff when loading up at speed.

Get a half ton pickup or a mid sized SUV and make sure you have trailer brakes.