I am trying to build a solar powered boat, and I have decided upon this 5V, 500mA panel. I can buy 3 or 4 of them to get up to the necessary voltage and amperage. The problem I have is that I can't find any good 5V motors; the closest are 6V, however as far as I know you have to match the same voltage exactly. I'm looking for the highest possible RPM and I want to spend less than $10.
A 6 Volt DC motor will function very well with a 5 Volt supply. A 9 Volt motor on the other hand will operate at around half the rated RPM at 5 Volts, and even slower with any appreciable load on the shaft. Therefore a 6 Volt motor is preferable.
Regarding solar panels, one would need to use multiple of the 5 Volt 500 mA panels in parallel, to preferably exceed the maximum stall current rating of the motor to be used. Stall current is typically much higher than the operating current. Thus, for a 6 Volt motor rated at say 1.1 Amperes maximum stall current, a minimum of 3 solar panels in parallel would be recommended, more if they are to be used in overcast conditions.
The question does not specify how much torque is expected from the motor, which is an important consideration for motor selection - speed ratings are typically at zero load or at some nominal load torque if this is explicitly stated.
For use with a propeller, a geared motor with lower RPM but higher torque would work better, for instance this one will deliver around 700 RPM with high torque, with a 5 Volt supply.
Similar motors can be found on many other sites - Radio Shack seems to be rather poorly stocked on such motors, so it would help to look further afield.
Edit to address updated question:
The 6 Volt motor does not seem to have current requirement specifications, but as stated above, it should work well enough with 2 of the 5 Volt panels in parallel.
To successfully supply the 9 Volt motor identified in the question, a series-parallel arrangement of 4 panels (2 x 2) will work - The panels will not be able to supply the full required current under load so the motor will slow down, but neither the panels nor the motor will suffer any harm.
I am sure that this motor will run using the solar panel. The bigger question becomes will it turn your propeller in water. This requires knowing the torque required to turn your propeller in water.
Linking enough solar panels together to get a higher voltage and current will probably be more successful however, without knowing the dynamics of your craft and propeller - everything becomes a guess.