On the R300, I have a primary and backup depth measurement. The primary is a 0-250 psig pressure transmitter (Digi-Key MSP6251P4-1-ND) tied into my PLC and touch screen display. This has worked very well as you can display the depth digitally and you can switch between sea water and freshwater in the depth calculation with a touch of the screen. As a backup in the event I loose both my main and auxiliary power supplies, I have a Weksler Altitude Gauge (BM43-YHT4-LW). The dial is for 0-450 fsw. The problem with a manual pressure gage like this is you introduce 2.5% error in the depth measurement when you move between sea water and freshwater because of the density difference. As with any pressure gage, you need to use a dead weight tester to calibrate the instrument.