Sims with this trait will get the negative "Misses water" moodlet, which has a -20 mood effect, if they don't swim for a day.
They also have their fun need go up quicker in water than Sims without this trait.
Sims can also get the interaction 'Talk about Water' with another Sim. If the Sim has the 'Loves Water' trait they will continue the conversation. If they have the 'Hydrophobic' trait, they will stop speaking to you and walk away. This also works for the interaction 'Talk about Swimming'.