You are correct to observe that the reduced power is the key to your flash's ability to freeze motion. More flashes may work (see caveat below) but since more gear usually costs more money, here are some alternatives

Slow the Event

If you are recording water droplets, record them right after they form. It takes a few feet for the drop to "speed up." If you are recording the splashes, use a smaller object/ droplet at shorter height and increase your magnification. The overall appearance is the same but the liquid will move slightly slower. Slowing down the object may allow you to increase the power and commensurate flash duration. At 1/4 power many flashes are still faster than 1/10,000s

More power

You could try a more powerful flash unit. It might seem counter intuitive but the duration of the flash is more closely linked to the relative power (I.E. A flash whose 1/16 power setting is 5 watt-seconds will likely have a shorter duration than a flash whose 1/4 power setting is 5ws, at those respective power settings.) This is not a hard and fast rule, though, as many factors in the design of the flash circuit play into the duration of the flash.

Multiple Flashes If you have ready access to multiple flash units and do not mind the visual effect of multiple light sources (perhaps combine them under one diffuser?) if they are perfectly synchronous. I've seen delays varying between 60 microseconds and 100 milliseconds in flash sync systems but a couple hundred microseconds is fairly normal.

Here is a method to check the synchronicity of your flash units:

1. Set up the various flash units next to one another and block most their light output. For example, use a piece of cardboard with pencil sized holes.
2. Place a lit flashlight next to the flashes.
3. Darken the room sufficiently
4. Open the shutter for a long exposure and move the camera quickly and perpendicularly to the arrangement of flashes. For example, if the flashes are side-by-side on a table, move the camera up and down. This can be accomplished by carefully swinging the camera in an arc.
5. Trigger the flashes while they are in the camera's FOV.
6. Once you have the shot (it takes practice) open the image on a computer
7. There should be several dots or short lines representing the flashes and one long line representing the flashlight
8. Draw a line along or straighten the image to the flashlight line
9. Draw a line perpendicular to the flashlight line, intersecting your master flash unit.
10. However far "behind" the master unit the slave units are represents the delay.

Notes:

• Any movement is sufficient so this can be done by moving the camera or flashes. I've seen people put their flashes on a passing car, for example.
• In a photo instrumentation lab this test is done with a camera equipped with a scanning shutter (many DSLRs can be used with a scanning shutter)
• if available, use a strobe light which is set to a known frequency so that the exact delay is calculable
• If performing this test with a rolling shutter, the movement must be perpendicular to the read-out motion.
• There are variations on this experiment which involve dropping a ball bearing past a ring of flashes. This works well to identify synchronicity differences but only if the object you want to photograph is slower than a falling ball bearing :)