pls tell me what sign conventions have to be followed in problems of vertical motion under gravity?

also, for river boat problems, boat takes shortest time to cross the river ;-

relative velocity of boat is perndicular to the river flow

or

actual velocity is perpendicular to the river flow?

please tell me about what is the level of questions asked in BITS as compared to AIEEE and IIT? is the competitions more than that of AIEEE?