From a thread at the Unconditional Parenting message board, where a poster asked about a situation in which a 3-year-old likes to splash water out of the bath all over floor/parents.
Get in the bath with the two children (between them) so you protect smaller child from tsunamis and you are warm and wet instead of cold and wet.
Put washing up bowls along the edge of the bath on the floor to catch splashes.
Buy one of those really cheap carpet mat things which have a plastic base (they look pretty grim, but they soak up lots of water, it doesn't go through to the carpet, and they can easily be hung on the washing line outside to dry off)
Cut up an old towel into strips and attach the strips to the side of the bath with strong tape. Drips going down the outside of the bath get caught in a towel strip rather than going on the carpet.
Make lots of little bottles of potion (food colouring and water) for pouring.
Get a waterwheel for pouring games.
Bubbles may be more fun to pat and throw around than actual tidal wave splashing
If your kitchen has tiled/lino/wooden floor, the sink may be a good place for maxi-splash fun (and you can get just as clean in a sink as in a bath). As long as child isn't in danger of falling on the floor, parent can use it as an opportunity to mop the floor with constant water provision!