I've played through Hoenn extensively with each starter (sometimes all at once), but my very first playthrough was Mudkip. In Gold my favourite had been Cyndaquil, and before that I'd picked Bulbasaur in Blue. So I don't know if it was a conscious decision, to pick a Water type after going through Grass and Fire. But something appealed to me about Mudkip more than the other two, and having basically skipped over Quagsire's existence in Gold, it was awesome to me that my Water starter was totally immune to electricity.
Since back in those days Brendan/May didn't use their starter in its final stage, it really felt like Swampert was all the more powerful, given the lack of many trainers who use Grass-type Pokémon. That's a bit of a sweeping generalisation on my part, I know, but literally the first time I felt Swampert had a weakness was after completing the League, facing a level 100 Bellossom in the Battle Tower at level 75 and instantly fainting to a Giga Drain.