Unfortunately, the waterproofing necessitates an enclosed battery. The 940mAh unit supplied lasts for less than an hour of shooting, and you won't be able to upgrade to a more capacious alternative. Nor would we recommend switching batteries underwater!
Being a Panasonic camcorder, the SW21 doesn't skimp on the manual settings, even if you're quite likely not to have time for careful configuration with such an action-oriented model. The Underwater mode remains, with its own discrete button. This applies colour correction to counteract the effects of shooting through water, as well as filtering the audio of motor noise, which will be much easier to hear thanks to the increased ability of water to transmit high-frequency sound.
Another button switches to manual mode, with a second press enabling manual focusing as well. Configuring these settings calls upon the cursor wheel under the LCD, and the process is about as easy as you could expect from such a system. Pressing down cycles through the options, with the other directions selecting them. You can configure shutter speed from 1/25th to 1/8000th, and independently adjust the iris from F16 to F1.8. Up to 18dB of video gain can be added once the aperture is fully open. The Soft skin, Colour night view and Backlight compensation modes are also readily accessible from this. The PRE-REC function can be toggled here too. This buffers a few seconds of video at all times, so if you're a bit late with the record button you won't miss the shot.
You probably won't need to enter the full menu unless you need to alter the quality setting or enable a scene mode. The usual five options are available here, including Sports, Portrait, Low light, Spotlight and Surf & Snow. Although extreme shooting conditions are the SW21's primary focus, Panasonic at least recognised the importance of Internet video and added a Web Mode, which has its own discrete button. This doesn't change the video resolution or data rate, but ends each recording when it gets to 10 minutes in length.