#6: It's Vibrating
The faster you try to go, the worse the vibration is. You might also notice the engine racing, while the boat loses speed.
Solution: Something's likely gone wrong with the prop. A nick or gouged blade can create imbalance and vibration; a towrope or fishing line can snarl the shaft; a direct hit on an object could remove or misshape enough metal to make the prop ineffective.
Sometimes a seemingly good prop might have enough unseen distortion or damage to cause cavitation and vibration. Short of changing to a spare prop - which isn't always possible or advisable when on the water - your best option is to slow down and concentrate on getting to shore.
If line - especially monofilament - has worked its way into the prop hub, you might have to trim up the motor until you can remove the prop and clean it out. Most outboards and I/Os can stand a bit of mono, but if there's enough to cause a noticeable decrease in performance, you shouldn't ignore the problem, as it could lead to permanent damage.
With outboards, the rubber bushing inside the hub can begin to slip and fail, causing a loss of power. Again, you might need to idle home.
Prevention: Consider carrying a spare prop, along with the necessary tools to make the swap. Practice changing props so there are no surprises, if you have to do it away from home.
Carry Onboard: Gloves to protect hand from prop blades and a brand-specific prop wrench.