you get HM surf
http://upokecenter.com/games/dungeon2/guides/wondermail.php
solar cave
I believe it is in Solar Cave...
I have a Lapras with surf.
1. get a water type Pokemon 2. go to solar cave 20th floor is where surf is
Any water pokemon. Goldeen, Barboach (etc.)
In Pokemon Mystery blue Dungeon, after you beat Rayquaza, you should be able to go to a dungeon called solar cave. There you will find the HMS surf, dive, and waterfall. In sublevels 10-20 you should find a few Chimechos. I have one Chimecho
Either use a key in solar cave or when lombre speaks of stormy sea, speak to whishcash
Normally its because the dungeon is big or if for exaple you have to get in with the hm surf and if no one can learn surf you could risk losng it, so yeah, the games just being helpful ;)
You use surf to access certain dungeons that are located in the ocean. You can also use it in battle to hit Pokemon using dive for double damage.
you have to get hm surf and then a spinda will apear then you can go to firey fields then beat that then you can go to lightning fields