I suspect that the depth is dropping out because it is genuinely not getting a reflection off the bottom. This could be because you are in very deep water (but it is unlikely that your slip is more than 200ft deep
) or, more likely, because you have marine growth around, or even inside, the through-hull.
I had a problem with speed always reading zero on my DST800, and it was marine growth jamming the paddle. I now have a round foam dish scrubbing brush, bought from the supermarket, that fits nicely down the through-hull. I give it a scrub now and then.
When was your bottom last cleaned? Even if it was cleaned recently, they can't reach inside the
Word of warning for later when you install the wind instrument : they normally have a terminating resistor, so get installed on the END of the network (in place of one of the terminators) - NOT as a drop.