watering tree is one of the least effective ways of making bff. Running team instance, group instance and group convoy with the same person EVERY day will get you to lv 5 friend real fast.
There are some other perks for high lv friend, watering could give more than 1 tree exp to each other for one thing(this is important on high lv tree where it takes 16/17 exp and everyone only have 1+10 watering, so in theory, if it's all 1 exp, some ppl wont get to harvest daily. It doesn't matter if you are really popular thou)
Also, the commission mission thing reward coin based on char level and friend level, so have a high friendship high level player friend will give you more coins.