3 chip digital in manual mode.

I have been using auto mode for a while and now I want to start using manual mode when filming from the boat.

In auto mode the shutter speed is 1/60, I want to get a cleaner shot but still want it to look smooth at full speed and when I edit it to slow motion ( usually 30-40%).

I was going to start at 1/250 and then adjust from there,
What do you think my shutter speed should be at?

thanks in advance