It looks like the PS-31 is the forward looking transducer and the PS60 ($4k USD) is the down looking transducer https://buy.garmin.com/en-US/US/p/503834#
The PS60 may have more receive elements than the PS-31, the Interphase sonar originally had an option for 3 x 8 RX channels.
The display looks very low resolution, probably do to using a standard multibeam algorithm on 8 RX channels.
I would be very surprised if Garmin released their communications protocol. Their commercial products tend to be closed systems. Their target market is pleasure crafts and fishing sonars which is probably much bigger than the ROV market, at least for now.
All of our sonars will have the communications protocol released so they can be used with any software.
On the ROV sector scan it sounds like something like a 675 kHz high speed scanning head would be more useful than a 300 kHz head.
Lets assume that for the ROV you would like to scan a 120 deg sector, with a range of 30 meters in front of the ROV. Taking speed of sound at 1500 m/s, this would give a maximum of 1500/(2*30 meters) = 25 pings /second.
Assuming a step size of 2 deg, it would take you 120/(2*25)=2.4 seconds for a sweep through 120 deg.
Would this update rate be fast enough?
You could decrease the step size for more resolution and/or decrease the ping rate for greater range. At 675 kHz maximum range should be around 50 meters. The beam width would be 1.1 deg for our 675 kHz transducer, giving the option for very high resolution sector scans.