I am 17 year old boy and i used to mesturbate alot before and because of that i was not getting an errection , so i stopped mesturbation and after 3 month today i did mesturbation and my sperm was watery i dont know why ,so kindly tell me what to do?

The semen(sperm) consistency varies quite a lot depending on the interval between ejaculations. It has no serious implications! So do not worry about it. Excessive masturbation can cause erectile disfunction, so go slow at it.

This might be a 'one time 'thing. Lead a normal life and avoid excess of any thing. You are young and leading a normal healthy life including moderate masturbation is OK and you will be fine