The HM surrounded by water in a locked room on the tenth floor is actually HM Waterfall, not HM Surf. If you have not lost the HM Waterfall Whiscash gave you, you can find a Link Cable there instead. HM Surf is actually on the twentieth floor and does not require a Key, although it is still surrounded by water.
In order to retrieve HM Surf, you will probably need to switch leaders mid-dungeon. This is only possible in certain post-Rayquaza dungeons, among them Solar Cave. Just in case you are not familiar with how to do this, designate the Pokémon whom you want to be the leader after clicking "Team" on the main menu and select the option "Leader." This will switch your control to that Pokémon. You can switch leaders once you arrive at the water surrounding HM Surf so that your friend capable of crossing water can retrieve the HM and rejoin its friends. Then, if you like, use the "Leader" command again to switch control back to your normal leader. Just be sure that the Pokémon you bring to cross the water is powerful enough to hold its own against the wild Pokémon in Solar Cave. Unless you are very confident in your skills to protect it, you may want to level up the Pokémon a bit before you enter Solar Cave. Keep in mind that HM Surf is found on the twentieth floor and does not require a Key to obtain.
Regarding HM Waterfall, don't worry about not having it. It can also be found in Solar Cave and actually does not require crossing water. If you unlock the locked door on the fifteenth floor, you can retrieve HM Waterfall without further incident. If you are really struggling to find a Pokémon able to cross water that is powerful enough to make it through the dungeon, you can obtain HM Waterfall first; carrying it will allow you to visit Waterfall Pond, a dungeon which, as others have said, is filled with rare and powerful Water-types.
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