I was quite surprised that this website seems to use a self-made map viewer. Maybe I'm just out of the loop, but it feels like by now everyone either just uses Google Maps and a minute, hardy fraction uses something based on OSM.
The increased pricing model Google Maps brought along a few years ago [1] makes it a lot less appealing for public-facing projects these days. It's still great for internal tools that have limited and predictable volume, but for anything public you'll both be building a cache layer yourself to reduce the costs as well as monitoring it closely. Something as simple as ending up on the HN frontpage could easily burn through your planned budget. Setting quotas only solves the problem so far since your service will still be unusable once you hit them.
Nautical maps (available, but not the default on vesselfinder) are usually a bit special. There's custom standards like ENC s-52/63/57 etc. Typically with a huge amount of symbol layers etc. Which is just one of the many reasons they may have gone with a custom openlayers solution
It could be a custom rendering of OSM data... whatever it is has full street grids for land-locked villages in the middle of France, which I can't imagine them gathering on their own. And their requirements (particularly for the vessel overlays) are peculiar enough that having their own renderer seems pretty reasonable.
Clicking [i] in the bottom right corner reveals this is indeed based on OSM data.
Kind of stupid design, as the [i] button is so big that a small always visible attribution could have fit better (just like on osm.org).
This is seemingly one of default options in OpenLayers (my guess based on how often I see this type of attribution). I wonder why people choose OL anyway. Sites using it in my experience have on average jankier UX than those based on Leaflet.
