Currently SeaSketch supports both OGC and Esri data services, listed in the Add a Layer interface (below). Only administrative users have access to adding map services to the Data Layers tab of the project.

Esri REST Services services are easily recognized by inspecting the URL. They often take a form that looks like this:

http://www.myserver.com/ArcGIS/rest/services/servicename/MapServer

or

http://www.myserver.com/ArcGIS/rest/services/servicename/ImageServer

By clicking on a map service URL, you should be able to read some metadata associated with that service. (Note, if the service is password protected, you'll need the correct username and password to view the metadata.) For example, the following map service is published by NOAA's CMSP data registry:

http://egisws02.nos.noaa.gov/ArcGIS/rest/services/RNC/NOAA_RNC/ImageServer

Clicking on that link reveals metadata that looks something like this:

If you can view metadata like this (with ImageServer or MapServer appearing at the end of the URL), you should be able to use this service in SeaSketch.