I assume your talking about while surfing. Just normal cruising any exposure to passengers should be minimal. You may have more when idling, but underway at cruising speed you should have very minimal exposure.
Now if your surfing and have the boat listed, there is the "station wagon effect" which can get more CO into the passenger compartment. It isn't too noticable, but a FAE is able to remedy this in any boat.
+1 about EFI and catalytic converters. Those minimize CO before it even comes out of the exhaust a lot.