bathymetric survey

Unless you are talking about rental, there is no such thing as a reasonably priced multibeam system. Of course, it depends on what you mean by reasonable.

A friend bought a second hand 'Geoswath' system and has been pleased with the results, although he's done a lot of the support work himself on fabricating poles, setting up the rtk receivers etc. Fortunately for him he has a background in electronics.

As far as I understand it the geoswath system is a form of multibeam which works best in water less than 50m deep. I've only assisted with providing base stations on the shore. Based on the results I've seen it's certainly possible to pick out small features on the seabed.

Yes, an interferometric system, such as Geoswath, does particularly well in shallow waters whereas the beam forming systems give better coverage in deeper waters.