Hi Sindre,
Unfortunately you wont "really" be able to extract the depths when they click around on the ENC, unless they click directly on a sounding specifically. One option could be to return the "Depth Area" value, this is a polygon area that represents one depth. Unfortunately since the ENCs only have the sounding depths recorded at the location of the sounding symbol, this will be as close as we can provide you. That being said, you could serve a bathymetric raster in the background as the depth source. This would provide you with a continuous surface by which the user could identify on. In this case I would be careful of the source of the bathymetric raster because if it differs in any way from the soundings on the ENC it could be misleading.
As for best practices for loading large volumes of ENC data. We have few options, first; the ArcGIS for Maritime: Charting Solution. This solution is designed to ingest large volumes of ENC data into a single database. It also allows you to calculate representations on the data and display the feature classes with S-52 symbology. Ultimately, providing you with a very similar look to flat ENC files. This solution is traditionally used for chart production, if this isn't really what you're looking for we have a couple other options.
The second option would be Maritime Chart Server, this is a Server Object Extension that allows you to place ENC's into a directory allowing a server to look at this location and render all of your ENCs with S-52 symbology in the form of a map service. This can be accessed in ArcMap, ArcGIS Online, or in a custom web application. Message me with your contact information and I would be more than happy discussing this further.
Finally, the last option is the S-57 Viewer, this is a FREE product that we came out with a couple of years ago. You install it onto ArcMap and it simply provides you with a method to add ENC's to ArcMap with the proper S-52 symbology. Check out S-57 viewer here.
I hope this was helpful.
Take care,
Lee
