The people who don't want to spend the money
on self leveling mount will find lots of reasons why they don't need it.
The people who don't care about spending the money
will find lots of reasons for buying
The science of the question answers the question for you.
The beam you are projecting from your radar is similar in many ways to a flat flashlight beam, if you want a large portion of your beam pointing into the water and up at the sky then get a fixed mount for your mast. Radar antenna
arrays are MADE to be mounted and kept horizontal for a reason. This question is best answered by speaking with the manufacturer, ask them if it is a good idea to mount the antenna
array sideways. See what they say.