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?

actually, could you give me advice on what ninjas i should save up to buy with coupons? i haven't been spending any of them just cause i'm a little afraid i'll regret it.

depends, can you show your team?

sure

talents are 32332 and shark summon

these are all the ninja i currently have:

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.

Or,maybe,focus on future BT ninjas,like Shisui kimono and sage Kabuto.These days BT GNW Madara dominate both Kakuzu and Kushina.

Dude why do u even ask what ninja should u get? Just work towards any ninja that u like in the anime or that u feel like u will enjoy the most.. dont be a meta chaser pls

for a good team use lighting main omoi sasuke and hinata great ninja war so use hinata as tank omoi so he can do damage and make clones and sasuke for extra damage and keep buffing lightning main because he is the main attack

Wait for a good event if you want a ninja from there spend atleast 300 coupons