It seems to be based on the rules of Water Polo. The size of the pool is between 20 to 30 meters in length. It doesn't specify how deep the goals are supposed to be, but the standard seems to be about 1.5m. So if you have a maximum size pool of 30 meters + two 1.5 meter long goals, then the total pool size should be 33 meters in length.
To answer why they chose 30m and not 25m since it fits within range.
While I don't have proof, I suspect it's because most competitive water polo is played in 30m pools. For example, the official rules of Canadian Water Polo Association (the only set of rules I could quickly find that doesn't require a PDF download) states:
The dimensions for games should be 25 metres (women) and 30 metres
(men) in length by 20 (not less than 17) metres wide. The depth is
ideally 1.8 metres or more.
So if the goal is to accommodate competitive water polo, you want to give the players enough space to play in the field in which they're going to compete in. Also, if you ever want to host a sanctioned water polo event, then you need a pool that fits the required dimensions.