Speed controlling of thrusters



We are trying to make ROV that would go 300 meters under water.It should have speed about 3 m/s.We are going to use six thrusters.Two for vertical movements and rest for horizontal movements.What kind of thrusters should I use?

I have searched on internet and I think that maximum thrust of each thruster should be atleast 12.5kg.But I am confused with the speed controlling mechanism of thrusters.How can I control speed of the thusters?