Creating bathymetry layer from i-Boating WMTS file

Longshot, but I'm hoping someone here has done this exact thing.

I work for a nonprofit and am creating maps for some of our local waterbodies. The only water navigation service I've found that has export of WMTS files is i-Boating. Their website looks like it's from 2001, and I've been unable to get a response from their support email about downloading files. The trial doesn't allow downloads and I can't justify the full app cost without knowing if the app is indeed still active and supported. 

The process from both Caltopo's and i-Boating's ends looks straightforward, but anyone who's done this, I'd appreciate knowing if i-Boating is deprecated or if I just need to be more patient waiting for a response. Or if anyone knows of bathymetry maps of US (specifically New England) inland waters that are available, either free or through a different mapping service, that aren't proprietary and can be loaded into Caltopo, I'd much appreciate it.