That actually depends on your personal opinion,

I mean since you're gonna be staying with the water main for a while I suggest you try to BT yugito ( 2 tails jin. ). To be honest the ninja you wanna get depends on the ones you have. But I'm suggesting to stack up your survival points to redeem a ninja you really want. However, I could tell you which ninjas to focus on using your coupons. Being f2p doesn't mean you can't have coupons can it?

Well I would suggest you to save up your coupons for fukrokomaru's 25k or 45k deals. Despite the fact that it takes a long time (over 5 to 7 months) , it's really worth it. You can gain a lot of initiative and obtain like 3-4 good ninjas. However, if you're not patient enough for that then start redeeming kushina ( red hot blooded habanero ) frags cuz she's really good and works well with all sorts of teams. After that try going for a meta ninja like edo minato ( for supporting kushina ) or earth grudge kakuzu ( for blitz teams). It totally depends on your play style. Judging from the fact that you prefer to use the water main I suggest you to go for a cancer team i. e. got for bt kushina. Although, if you prefer other mains then try the above.