The term "waterproof" refers to the material the speaker is made out of, not really the ability to act as a bulkhead. As Al mentioned, most speakers are made out of a cardboard like material. A marine speaker is not. They typically use a material that is flexible like cardboard but won't turn to mush when exposed to moisture. Mylar for example, polypropylene, although I suppose there are others. In addition, there are marine speakers that are specifically designed to function at reasonable distances from marine electronics that are affected by magnetic interference. Look for the designation "magnetic shielded". While I personally don't believe they will coexist within a few inches of a compass, they are supposed to.

There are also many, many brands of marine waterproof speakers besides Polyplaner. Almost every electronic manufacturer has a line of marine electronics. JBL, Kenwood, Bose, Infinity etc. You can find a selection at West Marine, but believe it or not, even Walmart carries marine speakers on their website. Frankly, the sky is the limit on marine audio these days. There are complete systems from the CD player to amps, subwoofers, separate dome tweeters, midrange and bass. I have had great success using the Pyle brand as do many of the people in the club. They are mid-priced and have quite a variety (try Amazon). You will also find a decent selection at a powerboat/waterski supplier like Overtons (try their website).

If you are really concerned about water intrusion, leave the inspection ports as-is and get box speakers instead. Once again there are several brands of these self contained speakers, once again sky is the limit on size and wattage capabilities.

i guess the bottom line is that you should not feel bound by looking only at Polyplaner, although they are decent. There are also alternatives to the type of speakers you have, even Polyplaner carries a line of box xpeakers. Hope this helps.