It's beyond the scope of this project to attempt to describe how to photograph opisthobranchs U/W; basically 'branch photography is the same as with any other similar-sized marine invertebrate: a reasonable amount of utilitarian and well-maintained photo equipment and perhaps the most important factor, a good amount of luck in obtaining the images. Possibly you might like to review Bernard Picton's information on finding nudibranchs for hints on how to search for and find these attractive and sometimes quite cryptic little animals.
In lieu of attempting to describe U/W photo techniques, I'll limit my discussion to what I currently use during and after SCUBA in obtaining the images.
I've successfully used a series of small glass aquaria in photographing the animals at home. The aquaria are made as follows: I use five sheets of 2mm thick glass stock per aquarium; the base is 250mm X 190mm and on top of the base are four 50mm high strips of glass cut to 165mm & 130mm, forming a rectangle. The five sections are bonded to each other by the use of a non-toxic silicon aquarium cement. The sharp glass edges are then sanded to reduce the possibility of an unwanted surprise. Once the aquarium cement has dried and the tanks leak tested, I then use a removable thin sheet of black (or any of several other colors) plastic which fits within the confines of the 'branch tank. With few exceptions, I like to shoot most of the 'branchs on this sheet of black plastic. The subjects are of course shot through the water's surface and not through the glass sides.
During photography I use a small, cheap, DC-powered light, designed for home video cameras. I've mounted the light on a small flexible utility stand and power it with a 6V lantern battery. This functions quite well as a "spotting light" to aid in focusing. I shoot the 801s using manual exposure mode, which allows me to manually adjust the aperture and I use either center-weighted or spot-weighted metering, depending upon the size of the subject. The 801s/105mm macro lens/SB-21 Speedlight combination is then used, making for a terribly effective photo system. If I have 'branchs below approximately 12mm, I then photograph using a Nikon bellows system (PB-6 & PB-6E) with the above parameters.
My close-up film preference U/W is Kodachrome 25 or Kodachrome 64; for seascape-type photos, especially with the Nikon 20mm lens (with the 801s) and the 15mm Nikonos, my preference is Velvia. My film preference for photos shot at home in the 'branch tanks is Kodachrome 200. This film, in conjunction with the SB-21 Speedlight, allows me to frequently shoot at an aperture of f/22 ~ f/64 . . . handy in terms of increased depth of field for shooting anything approaching 1:1 or greater magnification.
Once a suitable exposure has been acquired, the Nikon LS-1000 35mm Film Scanner is used to get the images into digital format. Adobe Photoshop 4.0 is then used to clean up the final images.
A good many years ago I used a Canon F-1/100mm macro lens w/Speed-finder housed inside an Oceanic Hydro-35 case. I used in conjunction with the housing, 2 Oceanic 2003 strobes (sometimes w/an Oceanic 2000 slave strobe as well). The strobes worked phenomenally well (while they were working!), but I was plagued w/numerous problems in having Oceanic service the units (long and unreasonable hiatus in correspondence, failure to correct the problem for which the units had been sent in for, long periods of non-response from the service department, etc.) I can't comment on the current level of professionalism by Oceanic, but I was treated so poorly in the past that it would be very unlikely for me to ever contemplate buying anything with Oceanic's logo on it. It's too bad that some US companies dealing in U/W gear and services can't emulate Ikelite. I've used Ikelite dive lights, strobes and other U/W photo gear and I'm absolutely delighted with not only Ike's products, but his service as well. Ikelite is extremely responsive to customer's desires and the company apparently takes great pains to keep customers very well satisfied. Quite a remarkable contrast between this company and several other companies that I've dealt with in the past.