A few more explanations regards to AI, default, potential conflicts etc…
As @CromUK correctly mention there is a difference between “leisure boats” and “ships and ferries”. Leisure boats spawns when you approach - mostly near to big cities - Ships and Ferries follow predefined routes.
None of it can really be considered AI…Artificial Intelligence is something else but the term was used for many years in simming for both planes and ships. Regards to Ships also no package provide real time shipping. Routes can be more or less realistic, but they are predefined. I was invited to work on an a project based on AIS real time data but I have so far said no; even with the amount of models we have available in the GAIST ships would still not be the real ships… We still miss around 300.000… 
And I have a full time job and a family…
Now to the potential conflicts between default and GAIST:
First a map of the default routes…
The chance of seeing a default ship is actually remote in most parts of the world… And in most parts there will be no conflicts even with default Ships and Ferries at 100%.
Next GAIST route net…
It is a different philosophy… routes are kept near to the coast where most people fly - for me something in the middle of the Ocean only serves to put strain on the computer… I just flew from South America to Europe in real life…didn’t spot a single ship before arriving over the English Channel so for me not worth simulating in the computer. That said the routes in the graphic here is as they were last year - quite a few additional routes have been added but still most along the coasts - but listening to a few people wanting traffic in the middle of the Ocean we did add some…
Next, so where are the conflicts…?
The map shows default routes - there are quite a few tiny ferry routes in e.g. Denmark and then some international routes. The green ones are overlapping with conflict as exactly the same route is included both in default and GAIST. In GAIST you get the real ferry on these…
In addition to the ferries you get a bit more traffic with GAIST in this area and then potential for conflicts increases as the ships are not intelligent and the more narrow the passage is the worse the risk of collisions.
The below is not the default ferry but Viking Line’s Cinderella in the Baltic Sea…
Then to the leisure boats, they spawn around your plane which is more performance friendly than adding them on routes. So having leisure boats on 100% could make sense. We are including more and more leisure boats in GAIST but using simple routes to keep performance, and only in areas known for having a lot of leisure boats like the Baltic Sea, the French Riviera, Greek islands…