I am applying to top BS’s in the North East, and most of them require multiple recs the than Math and English teacher. I only participate in one sport, Water Polo, and am not very close with my coaches. My teachers all like me, and would give me great recs, and my coach would give a mediocre rec. What would schools think of me if I only sent academic recs? Would it be better to send in my coaches rec? One of the teachers I would ask is the leader of a club I am in, so would this help?
Most top schools I know of only require 2 or 3:
typically academic recs from english and math teachers
1 personal recommendation of your choosing - optional/mandatory depending on the school.
You shouldn’t have to use the coach and I wouldn’t if it is going to be mediocre.
SwimKid did not get a rec from his swim coach. His coach doesn’t write them for boarding schools so we didn’t even ask. He got Recs from additional teachers.
I agree that you should not have to use that coach if you need a personal or extra rec. Perhaps you have had some involvement in a community service activity or religious group or volunteered somewhere and can ask for a rec. Also, nothing wrong with a favorite art or music teacher or even a play director or a boss/supervisor (if you are old enough to have worked for pay- even at a summer camp for example). Remember that schools are all just trying to get a sense of who you are -in what is a truly brief time period (even if it may not feel that way). Best of luck. 
Honestly just choose somebody that in one way or another is involved in some type of activity that you do, whether that’s the librarian from the library that you volunteer at or a teacher from a club that you are passionate about: who you get it from will not be detrimental. I got my extra/special interest rec from my piano teacher, and my personal rec from a family friend.