The thread size is 46mm. UR PRO filters are threaded on both sides, so you can put other lenses on top. I bought the green water filter when I first got my camera, but only used it once. The problem is, green water is also pretty dark. To shoot in natural light, you end up with shutter speeds in the 1/30 range at anything below about 24 feet, and then it's very difficult not to blur. Filters work best in tropical water in bright sunlight, at 40ft. and above.
Different subject. Just got back from a 3 day live aboard in the Channel Islands (Catalina was spectacular, with 70 degree water and 100ft. plus vis). Had a brand new SP owner on board-he had never shot it, and barely had it out of the package. With some assembly help from another diver who is Ikelite friendly, and a few minutes of set-up instruction on the camera from me, he was taking some excellent pictures. His only previous experience was with a low power p&s, and he said all his pictures were so bad, he wanted to get a decent set up. He bought a package from B&H with the SP-350, and Ikelite housing and an Ikelite DS-125 strobe and Oly Wide angle lens. Other than some blur outs from being too close with the WA lens and a couple of burns from having the strobe too close, this package took wonderful shots just shooting in "S" mode with manual focus at about 9 feet. He also had some nice shots in UW macro (though this is where he blew out a couple). The package is a little pricier than doing the PT-030 and a cheapie strobe, but with the results he was getting, it's certainly worth the extra cash if you have it.
