One of several problems. More than likely, your pump on that side lost its prime. Try putting the boat in reverse and backing up a few yards, then back in forward and see if this takes care of it.
If not, your shut-off valve may not be opening/closing all the way.
If it's neither of these, could be the fill line is too long. Cut it to fit and re-install.
"Things turn out best for those who make the best of the way things turn out" - Zachary Troy Schrah - a young man with vision far beyond his years.