I know about the regulations but I was thinking of additional sound signals that are only permitted, not required might be a good idea for electric vessels.
I.C or electrically powered road vehicles used to both simply be powered vehicles, this was when electric ones were less common and no electric car model was commercially available.
Now, newer regulations treat electric road vehicles a little differently, so how about New Coast Guard regulations that treat electric vessels a little differently from other powered vessels?
At the moment, most powered vessels have internal combustion engines, and maybe nearly all did until recently, similar to road vehicles. And as noted before, one can tell by ear that propulsion is being operated. This would be why the regulations don't (currently) require someone operating a powered vessel to signal they are operating fore propulsion.
It seems that being able to tell by ear if propulsion is being operated could be useful. Say you are sailing and a powered vessel is coming up behind you. If it is altering course, a horn signal is given, but what if it isn't altering course?
Or what if a powered vessel is coming around a blind corner, isn't it useful to hear it's engine?
And maybe the engine sound indicating that forward propulsion is being operated may even be of use if the other vessel can be seen. Just because it is moving forward doesn't mean that propulsion is being operated in that direction, especially given the dynamic properties of water.
When out at sea, it may be hard to judge the ground speed of a vessel.
Electric boats may well be quiet enough that others might not be able to tell (without a sound signal) if propulsion is being operated. So maybe a bell sounded when operating propulsion in the other direction might be useful.