concept2 are the ones nearly all gym’s use, and for a good reason, they work, they dont fall apart, they feel (relatively) realistic, reasnobly quiet, adjustable resistance, and the computer readout has enough functions to keep anyone happy, especialy the newer ones with the force/time graph, very usefull for identifying where weakeness’ in your stroke are in real time. Should show a cross between a rectangle and a semi circle, maximum force when the oars are perpendicular to the boat, but wide enough to be doing a meaningfull ammoutn of work. Avoid a little hump forming at the end, thats your back muscles pulling your shoulders back and can give problems.
Had a brief go on one of the water ones a few years back, feels good, nicer noise, but lacks the C2’s computer and the price tag makes you think ‘HOW MUCH!’