You need to brack the cracks with bubbles coming out. Hope this Helps!
some are- like the gulf stream and rip currents. but others are not strong. it depends on the location and other variables.
Well you can either go through the Piranha's by feeding them every so often whilst proceeding through, or, you can simply swim around them on the far right, which is easier and is what I personally...
Surface current are formed by surface winds (not in the ocean) but deep ocean currents are affected by thermohaline circulation (in the ocean).
The song is River of Dreams by Hayley Westernra.