Six months ago, I changed from a D200 in a Subal housing to the OM-5 in a Nauticam housing. I have shot the 45 degree Inon viewfinder in an Ikelite housing and the straight Subal sport viewfinder with my Subal housing. I feel strongly that these viewfinders were necessary for the older, non live-view digital SLRS.,,,for shooters like me with old eyes.
However, I would recommend against buying any viewfinder for the OM-5 in the Nauticam housing for these reasons:
1) These viewfinders are very large for this small of a housing.
2) I feel they are not necessary because: 1) I find myself shooting in the "live view" mode more and more; 2) The live view viewfinder is really large, compared to the D300 and D200 view screens; and 3) More importantly, the automatic focusing mechanism has been very sharp. I find myself not worrying about having to actually visually confirm that I have focus.
3) Both of these viewfinders are made to be added to the Nauticam housing very easily by the user if you want it later. This is not true of the Subal or the Ikelite housings--these must be added by someone who knows what they're doing. I would try it first and see if you feel you really need it. You can always buy it and add it yourself later.
If you need a viewfinder added, I would go with the straight one. The advantage of the 45 deg viewfinder is that you can get really low for macro. The big disadvantage is that when you're following any fish that's moving, it will be hard and un natural with the 45 deg viewfinders. I never got used to shooting pelagics with a 45 deg viewfinder. I'd settle for having to get on my stomach for the macro shots in order to not miss the exciting pelagic shots.