Finding a speaker isn't a problem. It's finding room to mount the speaker, near enough to my head. Below is a picture of my dash, the VHF is not visible, but it's lower than the wheel, to the right, well below my knees. That's about the only spot where there is room for the speaker ( I know, horrible helm design), so when I'm standing, the speaker would be about six feet from my ears, or the same distance as the vhf. It's hard to beleive that a bigger speaker, mounted in the same spot, with the same wattage, would make that much of a difference.
I'm going to be honest and say this, at the risk of getting myself flamed, but I don't really care that I can't hear other calls when I'm running at cruising speed. In ten years of boating, I haven't once heard a hail that interested me, and I've heard plenty of hails - while sitting behind the windshield, running at slower speed, etc. I mounted a permanent VHF, and carry a handheld for three reasons, in case of emergency on my part, to keep abreast of weather info, and because I find the chatter to be soothing when I'm drifting or at anchor. That's about it. I know I'm required to be listening all the time, and in a legal sense I guess I'm complying since the radio is on - but the idea of fitting my boat with a larger speaker, and possibly an amp, so I can blare my VHF at 110 decibels to hear a bunch of people asking where the fish are biting, and for radio checks, and to hear the coast guard try to track down a ten hour old mayday call that was probably a hoax, while I'm running at 45 mph, isn't very appealing to me. But I do worry that I may miss an urgent hail from someone else, who could use my assistance. But of all the times I've helped others ( I've towed boats, towed waverunners, towed a boat that was on fire and then helped put out the fire, helped push off people stuck aground, helped people get their boat into the fuel dock, helped untangle a ski rope from a jet drive) of all these times, not once was I repsonding to a VHF hail - everytime it's been somebody without a VHF waving their arms.
Here's another question, could be a poll. Has anyone ever received a hail, on channel 16 or any other, that required their assistance, to the point that you, or they, were darn glad that you had the VHF turned on? Or any other hail that you were glad you didn't miss?