This issue was resolved as of the 1.4.6 update, and I am also now using Optifine without problems. I am convinced by Marcus' troubleshooting that the problem was somehow caused by Optifine 1.4.5_HD_U_D3, despite my attempt to do a clean vanilla install. From the optifine forum, there is an edit that says OptiFine 1.4.5_HD_U_D4 "fixed monster spawning issues"
I added 2 pictures from my Singleplayer survival world, which I've been playing in for about 2 months. Note the low entity count in the ocean biome, even after I stood there for several minutes.
This issue was resolved as of the 1.4.6 update, and I am also now using Optifine without problems. I am convinced by Marcus' troubleshooting that the problem was somehow caused by Optifine 1.4.5_HD_U_D3, despite my attempt to do a clean vanilla install. From the optifine forum, there is an edit that says OptiFine 1.4.5_HD_U_D4 "fixed monster spawning issues"