Old thread I know, but just wanted to put some info here about boating on this river and some of the ramps.
I would never use a motor with a prop, jet boat only. Also, you'll need at least 600cfs on the gauge at Jug Bridge for venturing very far. 800 would be preferable. Above 1200 and you're dealing with some pretty strong current right at the ramps.
I've launched at 4 of the ramps and fished the areas.
Starting at the most downriver:
Mouth Of Monocacy Boat Ramp - This is a good ramp and a really good fishing area. You can go up the river about two miles or so the the Rt 28 bridge where there's an impassible rapid. Below that rapid is a very good 1/2 mile stretch of water for bass. You can also go out into the Potomac if the Monocacy is too low or muddy.
Park Mills Boat Ramp - Another good ramp, but it's a little narrower. Here you can go down the river to the Rt 28 bridge rapid. Wouldn't recommend going down the rapid because you're not getting back up at most levels. There are two cobblestone rapids at lower levels, but over 800 you'll have no trouble getting up and down them. At 600 you might have to drag your boat up one of them(not fun by yourself). Lots of great fishing through here. You can go up river to the Greenfield section, but that is a mess of rapids and boulders. Park you boat and get out and wade! When it's high enough to pass, the current is horrendously powerful, and you will wind up sucking gravel into your grate and clog the drive.
Riverside Park Boat Ramp - Nice, big ramp, but often choked with mud deposits. You can go up the river pretty far, at say 800cfs, maybe up to Biggs Ford. I've only gone up to Rt 26. Lot of good fishing. I have not gone down river for fear of not getting back up. There are some deep holes in this area above the ramp.
Creagerstown Boat Ramp - This ramp is very narrow. You can go up the river, but there is an island not too far up that causes issues. The channel on the right is passable, but it's narrow, and has a LOT of current even at low levels. Once past that, you can go up along a very long, deep pool. Haven't seen how far I can go beyond that. Here I think you need at least 800cfs, and I'd be leary of getting back up unless you've got a buddy to help, as there is a cobblestone rapid about a half mile down. I haven't gone past that by boat, but wet waded several times, good fishing below the rapid, and then the river gets really deep about another mile down.
I have not tried out Pinecliff Park Boat Ramp.