What are the positives of mounting your carbs perpendicular?

Easier to change jets for one thing. I would be interested in hearing what some of the more knowledgeable people say regarding fuel slosh differences.

Clearance, I don't believe you can fit double pumper holley's end to end, or any holley with a center hung fuel bowl. This is assuming you are mounting two carbs on a tunnel ram.

I have a single carb on each engine. I am wondering if there is an improvement because of the angle of the boat sitting in the water.

On a single 4bl application it would not be recommended to run the carburetors sideways. It will lead to distrabution problems when you're at part throttle. One side of the engine is going to see more air and fuel than the other side. This is going to lead to bearing problems as well as other longevity issues.

If you've got a dual plane manifold with plentum split, it's probably a really bad idea.
The only thing I've seen them on are twin carb applications on open plentum manifolds, tunnel rams, etc. And I think that is for carb bowl clearance issues.
The only other application is on blowers. Again clearance is part of it, but also fuel distribution (front to rear) is supposed to be more even throughout the rpm range.
That's what I've heard.

I doubt the motor would even start like this on a dual plane. Only 4 cylinders would get any air/fuel, and the signal would be 1/2 (per barrel) than normal

Well..........
There you go.............
A really bad idea!