For just running around the lake, the stock bilge is adequate, if you start to do tricks and submerge your ski then your hood will allow a certain amount of water into the engine bay. Factors such as hood seals, breather holes, time under water, frequency of subs, will all determine how much water gets in your engne bay. You will want to size your bilge setup correctly to eliminate the water in a timely fashion so you don't suck any water into your carbs.
Your riding style will determine your bilge requirements but I feel this is one area where more is always better. You can always turn them off if you don't need them but it's impossible to turn them on if you don't have them at all.
You also have options with Scuppers but that's a different topic altogether.