Water absorption can occur before you pump a drop into your tank and no fuel system is isolated from the atmosphere. If it was, you'd have a vaccuum in your tank as soon as you started riding. Outside air filling the void in the tank as fuel is consumed may be filtered through a carbon canister, but that doesn't remove humidity.
If you have a fuel injected engine with a closed loop system that was designed to run on E10, it can probably accomodate E15 without too much trouble. If the design was marginal and you were maxing out the trim, fuel injector flow or pump capacity with E10, E15 may push you over the edge. Open loop and carbureted engines may require modification to run properly on E15. Unfortunately, those modifications may be illegal.
Fuel pumps, injectors and intake valves are lubricated by fuel. Ethanol provides less lubrication than other gasoline components and may reduce component life as the percentage is increased.
E15 isn't an absolute evil, but it will cause issues for some engines. Don't assume that if an engine was certified to run on E10 that it was optimized for it. It may have already been on the edge at 10% and 15% ethanol may push it over.