If you have easy access to the batteries, maybe run a little experiment. Once they are discharged under their normal conditions, remove both of them. Then hook them up in parallel with an amp meter on the high side. Place the positive lead of the Amp meter on the pos terminal of the battery with the highest voltage and the negative lead of the Amp meter on the lower V battery. (Maybe place a 10Amp fuse inline as well). As soon as you make the last connection, you'll see the exact Amp draw being transferred from the high V to the other, as they balance out.
Note: I chose a 10A inline fuse because most off the shelf multi-meters will only test up to 10A's anyway. Put the fuse before your DMM on the + side, this way the inline fuse will blow before any current gets to your DMM. Like Naruis said above, since the differential is only 1V, I think you'll find the current won't be very high during the transfer. The exact number is dependent upon the batteries ESR, but unless its a crazy high Ah battery, I think you'll find the energy transfer is far less dramatic. Also, when the boat relay triggers, you have an external source, your alternator, providing current, which means even less has to come from battery A.