Would a double pole 30 amp, 120-277 volt spec grade switch be a suitable disconnecting means for a 240 volt, 5500 watt storage type water heater.

Yes, it would. What it would NOT be suitable for would be for two different legs of a 480 volt system.

Call me crazy, but the last time I checked, a 2P30 amp switch (spec. grade obviously) costs more than an unfused air conditioner disconnect. Not to mention having to man-handle four or five #10's into a device box.

There aren't many 1g device boxes big enough for 5 #10s and a device (17.5 cu/in) I would use a 4x4 with a ring.

That's pretty much what I meant, Greg. Even in a big box, that's still a bit of work. I don't like to work any more than I have to.

It appears that many 30/60A disconnects use the same housings with different guts, so a 30 amp pullout disconnect is chock full of room.

I had to wrestle with one of those toggle "motor starter" switches in a DEEP box last week in a block wall. It was still enough to break a sweat to get it put back together.

Most boxes I have use are 22 cubic inches in volume.

Come guys. You don't let some #10 wires hand you your hinnie to you, do ya'? I thought you guys were big bad sparkies.

When I rough in a 30A for a dryer I'll use a 2gang box rather than taking the time to wrestle with a 1gang box work. The extra 30cents doesn't seem to be worth the extra work to me.

I would do the same for a water heater (where cord/plug are allowed) or use NO box and a cheap disconnect like EV6something mentioned.

Why make anything harder than it has to be?