There’s a lot to say on the topic. Your sensors setup is only seen on very large passenger boats, which justify the extreme costs of the equipment and computing systems required.
I have been involved in a few R&D projects on marine ASV collision avoidance systems and my original idea was exactly a blended Radar + forward looking sonar + AIS; however my supervisor turned it down due to the excessive costs (just the forward looking sonar is priced $3k+) and useless data produced.
In fact, being the radar only 1 or 2m above the sea level, readings would be excessively noisy.
I also checked with most radar producers and, even when removing all unused parts and replacing the chassis with thin fiberglass, the weight is still above 15Kg and 50cm diameter, and I’m pretty sure until last year there was no solid state small radar sold off the shelf (only military grade SAR radars unavailable for universities and individuals).
So to sum up, a simple (thermal) camera, terrain/bathymetry map and AIS receiver are more than enough to build up a reliable collision avoidance system for a small to medium sized boat/sailboat.
Going to my implementation, I though about having a very basic MAVLink message with MMSI id, positional data, heading and speed info and vessel dimensions, in order to create a vector field with predicted routes and perform a basic path planning.
You can check my current implementation here: