As seaducer mentioned some G10 case allow the use of the internal flash, some do not. However, the internal flash is only really useful for close-up photos. Small particles suspended in the water scatter light, so flashes cannot light as far as on land. These particles also reflect light back towards the source. This can mean distracting bright dots in photos. Using an external strobe reduces this effect.
For the cameras mentioned, do I need to buy an external strobe? For example for the Canon G10 or for the DC800?