Welcome to CF.
I have only visited Tunisia
briefly by yacht in 2007 (Hammamet), but I'm chiming in as there seems to be a lack of responses. Visa renewal is easy to accomplish if you know where to go (in Hammamet it involved a taxi from the marina and a purchase
of stamps beforehand). Best to get some local knowledge about this when you arrive.
There were dozen of liveaboards from many nationalities there at the time. The marina was near new, well set up with good security
(a guard on each pontoon and the whole community was gated).
A huge bonus was the mild temperature during winter - 20 odd degrees most days.
There were some some some excellent bargains in the adjacent 5 star hotels that many guests of cruisers took advantage of.
Hope this is of some help.