26 feet should easily be enough to get you to the rear deck of the boat, next to the guards. The one facing away is a bit farther away than the one facing you, but close enough you can still see his head. You should also be able to attack. Which actions and options you have available depends on what you do, but you should be able to make it. However, you'll probably need to use both the bonus Dash and move to make it all the way up including the 10-foot move along the boat, which is, again, doubled due to difficult terrain. You'll have some movement left, however, thanks to your unarmored movement speed increase.
@Michael: Splint mail takes 5 minutes to get out of, and 10 minutes to get fastened properly on again. You could probably get it loose enough to retrieve the rings in about half the time.