[onewhippedpuppy]

That's actually not true with regards to bilge blowers, you just need to search specifically for continuous duty blowers, not intermittent. Here's an example that I found in about 5 seconds: https://www.hodgesmarine.com/Jabsco-4-Flexmount-Continuous-Duty-Blower-p/jab35770-0094.htm

As for sound, you just need to match the ducting and fan accordingly. Bigger ducting will cut down on the negative pressure but also make it quieter. You can also pick a lower speed fan to cut down on sound. Another option related to the above fan that is 24 VDC, if you run it at 12 VDC but keep the same current it should cut the RPM in approximately half. There are an abundance of options and they are designed for this exact application, I don't think you should rule it out.

If you want to go in a different direction, maybe a household bathroom exhaust fan would work? They are designed for continuous duty and would be quiet. I would just be concerned about how they would hold up on a boat, and you have to have AC power available.