looking at Today's NHL Rosters, this is a list of player that surely would be eligible to play for (even if many of them have already played at the World Champs/Olympics for / in the past, sometime recent past)
Forwards
Adam Fantilli (Columbus)
Kyle Palmieri (NY Islanders)
Noel Acciari (Pittsburgh)
Philip Tomasino (Pittsburgh)
Andrew Mangiapane (Washington)
Sonny Milano (Washington)
Joe Veleno (Detroit)
Michael Pezzetta (Montreal)
Michael Amadio (Ottawa)
Anthony Cirelli (Tampa Bay)
Max Domi (Toronto)
Max Pacioretty (Toronto)
Tyler Bertuzzi (Chicago)
Ryan Donato (Chicago)
Nick Foligno (Chicago)
Matt Boldy (Minnesota)
Marcus Foligno (Minnesota)
Luke Evangelista (Nashville)
Michael Carcone (Utah)
Matias Maccelli (Utah)
Alex Iafallo (Winnipeg)
Cole Perfetti (Winnipeg)
Gabriel Vilardi (Winnipeg)
Robby Fabbri (Anaheim)
Frank Vatrano (Anaheim)
Troy Terry (Anaheim) ???
Matt Coronato (Calgary)
Anthony Mantha (Calgary)
Macklin Celebrini (San Josè)
Ty Dellandrea (San Josè)
Tyler Toffoli (San Josè)
D-Men
Dante Fabbro (Columbus)
Brett Pesce (New Jersey)
Victor Mancini (NY Rangers)
Brandon Carlo (Boston)
Ben Chiarot (Detroit)
Steven Santini (Tampa Bay)
Mathew Dumba (Dallas)
Nick Leddy (St.Louis)
Robert Bortuzzo (Utah)
Sean Durzi (Utah)
Alec Regula (Edmonton)
Cody Ceci (San Josè)
Mario Ferraro (San Josè)
Alex Pietrangelo (Las Vegas)
Goalies
Drew Commesso (Chicago)
Joel Hofer (St.Louis) ???
but I'm pretty sure that there are a few more with Italian heritage coming from their moms and/or wives (Canadiens' Josh Anderson, for instance)...
it's about 2 main factors: first, the pool's size and shape (depth, width and also the gradient of the pool's sides -if the walls are perfectly vertical or not), which has an influence on the water mass movement following the swimmers' action (the more the walls can absorb the waves made by the swimmers, the faster they can go; meanwhile, the bigger the waves are, the more they slow the swimmers down); second, water hardness might also be a factor (less than the pool's technical features, but still) as in different Countries there might be different habits (if not laws) about the sanification of the water itself (for instance, here in Italy we use to put a lot of chlore in pools' water, more than in many other places)
environmental and water's temperature can also have an impact, but in the indoor events I think there's no big difference between places (and the water's temperature is standardized by FINA for sure)
altitude might also be a factor (as it is for all the aerobic efforts), but normally there are no swimming big events in high places, so it's more a chance than a real thing since the 1968 Olympics and the 1975 Worlds (which were held in Cali)