Sure, but I'll preface this by saying that I think the original assertion that the enemies in the first three levels of Swimmer were borrowed from Mario Bros. is flawed - it was the other way around.
In level 1 of Swimmer, the first enemy is a turtle. Level 1 of Mario Bros. has the Shellcreeper, which is also a turtle.
Swimmer's second level has crabs for opponents; Mario Bros.' second level has the Sidestepper, which is also a crab.
I have no clue what the third enemy is in Swimmer, though it definitely resembles a fly much like the Fighterfly in level 3 of Mario Bros.
Where I say that Swimmer borrowing from Mario Bros. was flawed boils down to release dates: Swimmer came out in 1982, Mario Bros. in 1983. If anything, Mario Bros. might have borrowed its initial enemies from Swimmer.
Now, I can see where Mario Bros. took some inspiration from Joust, which came out in 1982, a year ahead of Mario Bros. However, Joust 2 (1986) was basically a rework of Vs. Balloon Fight (1984), so it all came full-circle in the end. Well, except for Swimmer, which has some of the most annoying in-game music I've ever been subjected to.
Circle of life, man. Circle of life