At my first IM I had to pee constantly.
I have basically solved that problem by following measures:
- stopped drinking a lot the day before
- just a coffee on race morning, no more drinking.
- powerbars instead of gels the first hour on the bike: no drinking.
- after that I drink when I'm thirsty, but no plain water: plain water is not taken well by the body and "runs through". So just electrolytes, you have to take which the organization offers you at aid stations.

Following point is also worth noting:
After an intensive training unit (especially running) the body stores water. I read once you can even measure that by measuring the diameter of the thighs. Because of this, the urge to pee is less for some (one or two) days. After regeneration (these one or two days) the water leaves the body via peeing. I experience that such that I have to pee constantly (with a light colour) during a couple of hours. What we learn from this: don't train intensively some days before a race. But: having a (smaller) intensive run the day before can work positively on the pee problem the day after.