"...we would like to stop somewhere between to check out a castle or a boat ride. We will be traveling with some heavy luggage..."
The luggage problem is a substantial one for this area. The castles involve a steep walk or additional transportation - and a boat ride is logistically challenging as well, even without luggage. But you CAN do this one way or another as long as you plan your visit with the logistics in mind...
(I assume you're traveling by train in the first place.) Other TI offices besides those two will hold bags too. I know Braubach's does. Check with th offices to find out and to get their hours - here's a list with contact info: http://www.mittelrhein-wein.com/Links/links-orte.html
(BTW, Braubach's Marksburg Castle is the only intact medieval castle in this area - and an excellent choice.)
But if the TI's are shut, well... And of course you have to schlepp the bags to the TI from and back to the station, then schlepp them some more once you're in Koblenz.
So the better solution IMO is to book in one of the charming Rhine villages rather than mostly dull Koblenz. (The other problem with Koblenz is that the hotels near the station are a bit dreary; getting to the better part of Koblenz will normally mean a bus transfer or taxi ride once you arrive at the main station because of the distance.) Drop the bags first at the front desk in one of the villages, and then you can head out wherever you like and come back whenever you're done without racing back to the TI before 5 pm or whatever. You can proceed into Koblenz the next morning by train.
If you stay in tiny St. Goar, for example, besides the best scenery on the Rhine, you'll find many good lodging options near the station and the boat dock. This map shows how snug the town is:
http://www.st-goar.de/files/stadtplan_st_goar-parken.jpg
St. Goar is a wise choice as well because of RHEINFELS CASTLE and because of its location right in between the other desirable towns of Boppard and Bacharach (and Oberwesel right next door.) You can be in any one of these towns by train after a 10-min. ride.
If you are going to take a boat ride... make the best use of your time; the most scenic cruise is done DOWNSTREAM, beginning in Bingen and ending in St. Goar (1.5 hours.) You want to be rid of those heavy bags before boarding, IMO. (Round trip boat travel is tediously time-consuming - use the train instead for one leg of this outing.) If you book in Bingen or in St. Goar, you simplify the logistics. Koblenz is well north of the main scenic zone, which you can see on this map:
http://www.bingen-ruedesheimer.com/images/map/rhine-valley290.jpg
ONE MORE OPTION if you just have to stay in Koblenz for some reason: Ride the train right through the Middle Rhine Valley to Koblenz, drop bags in a station locker, and backtrack by train for the rest of your day. Braubach (Marksburg) is only 10 minutes south. Boppard is only 15. Both are really nice towns. Perhaps with an early start that day you'd have time to see St. Goar (Rheinfels) in addition to Boppard - both are on the same side of the river.