Hello Jan,
i write you in english instead of german to make this discussion public.
I already wrote you in Wetpixel regarding the Ikelite Housing and the domes.
I have a Canon G12 in a Patima alu housing and i made some of my best pictures with it as the G12/S100 have a zoom and macro lens
and this avoid the frequent situation that you have the wrong lens for the things you see.
As i understand you don't want/can't to spend a lot of money on a housing for your D5100 and lenses, so i strongly suggest you to go with a
G12 or S100 rig and with the money you save you can buy:
2 strobes (Inon s-2000 or D-2000) including tray and arms
1 wide angle wet lens giving you up to 165° field of view using the INON UFL-165AD bajonet mount fisheye lens:
INON Underwater Fisheye Conversion Lens UFL-165AD [Overview]
This lens seems to work only up to 165° with the Fix-Fishey hosuing for the S100 as only this housing comes near enough to avoid vignetting or zooming.
The second reason for a advanced compact camera is the size,
with a DSRL rig you will need most probably to buy a extra luggage for your dive holiday as the size is too big to have your camera and housing stuff carry-on.
I recommend DSLR rigs only for professionals and/or photo enthusiasts with a thick, well filled wallet to afford the price, strong arms to haul another 15 kilos of gear around airports
and nerves like steel during the flights when the rig is out of your sight.
Chris
P.S Try to contact NEMROD here, he is the S90/100 guru in this forum and had a s90-fix fisheye - INON UFL-165AD bajonet mount fisheye lens shooing beautiful pictures!