The Seatool XSi housing is available in three viewfinder configurations, one of which is perfect for your shooting style:

Optical Window Viewfinder This Optical Window Viewfinder is standard on the XSi for convenient 1:1 viewing of the camera viewfinder. Inon 45° Magnifying Viewfinder The 45° Inon Magnifying Viewfinder is great for keeping the photographer up off of the reef while shooting low angle macro subjects. The 45° Inon Magnifying Viewfinder can be rotated 360° which is useful for shooting in portrait orientation. Inon 180° Magnifying Viewfinder The 180° Inon Magnifying Viewfinder offera a magnified corner to corner view in a traditional configuration.