If Mycobacterium ulcerans is derived from Mycobacterium marinum it is possible to consider the hypothesis that many epidemiological aspects between these mycobacteria may be in common :for example the modality of transmission to the man.
There is a clear explanation in this site regarding the modality to infect the man by Mycobacterium marinum .
It is possible to apply more or less these information to Mycobacterium ulcerans.
From this site
The main difference between Mycobacterium ulcerans and Mycobacterium marinum is that Mycobacterium ulcerans produces a toxin (Mycolactone) which destroys tissue and suppresses the immune system.
See this post