Dahab is mostly very safe but as any city in the world there are some places not safe especially when ur walking alone, Dahab is also very nice for shore diving lots of dive spots. there are centers that can rent you equipment if you want other centers do a package including renting equipment and guide and transport.
and there is no limit on your diving here only your air consumption and RDP tables and expertise these what limits you. most guides will be happy to stay underwater as long as you can

you perhaps want to get your own wetsuit or buy it from any local shop in dahab very cheap usually cheaper than online, as u dont want to use rental ones otherwise stick with a reputable dive center.
I am actually planning to go live in dahab in the next couple of weeks or so, perhaps if you would like u can contact me and i ll assist you in choosing a diving center and accomodation as well for free

haha
i forgot to mention Dahab is not very expensive to enjoy ur time over there