Players will likely recognise the Gyarados in particular from their time with Pokémon Gold, Silver, and Crystal in the main series: red Gyarados is the first canonically recognised Shiny Pokémon, appearing in the anime series as well as at the Lake of Rage in a scripted encounter within the games.
There's also a snazzy Magikarp hat for your avatar, which is free to wear and makes you look rather dapper or something less complementary...
The Water theme ties in with World Water Day on 22nd March (thanks commenters!) - but it seems as though this was also something of a way to sneak the two Water-type Shinies into the game at the same time, for the die-hard fans to discover.
The Water Festival itself will bring an increased number of Water-type Pokémon to the game for a limited period, finishing on 29th March, and working in a similar way to the Valentine's Day and Halloween events that went by over the past few months.