Fixes fishpeople swimming slow in deep water (#91132)

## About The Pull Request
Instead of having the deep water tile slow people down, they get slowed
down by the swimming status effect. Fishpeople are immune to that
slowdown.
## Why It's Good For The Game
glug
## Changelog
🆑
fix: fixed fish infused people swimming slow in deep water
/🆑

code/__DEFINES/traits/declarations.dm

@@ -189,7 +189,7 @@ Remember to update _globalvars/traits.dm if you're adding/removing/renaming trai
 #define TRAIT_NOBREATH "no_breath"
 /// Mob doesn't take oxygen damage in deep water
 #define TRAIT_NODROWN "amphibious"
-/// Mob doesn't take stamina damage from deep water
+/// Mob doesn't take stamina damage from deep water and doesn't get slowdown from swimming
 #define TRAIT_SWIMMER "swimmer"
 /// Mob is currently disguised as something else (like a morph being another mob or an object). Holds a reference to the thing that applied the trait.
 #define TRAIT_DISGUISED "disguised"

code/datums/elements/swimming_tile.dm

@@ -70,9 +70,12 @@
 	. = ..()
 	stamina_per_second = ticking_stamina_cost
 	oxygen_per_second = ticking_oxy_damage
+	if (!HAS_TRAIT(owner, TRAIT_SWIMMER))
+		owner.add_movespeed_modifier(/datum/movespeed_modifier/swimming_deep)
 	RegisterSignal(owner, SIGNAL_REMOVETRAIT(TRAIT_IMMERSED), PROC_REF(stop_swimming))
 
 /datum/status_effect/swimming/on_remove()
+	owner.remove_movespeed_modifier(/datum/movespeed_modifier/swimming_deep)
 	UnregisterSignal(owner, SIGNAL_REMOVETRAIT(TRAIT_IMMERSED))
 
 /datum/status_effect/swimming/tick(seconds_between_ticks)

code/game/turfs/open/water.dm

@@ -79,7 +79,6 @@
 	desc = "Less shallow water."
 	icon_state = "deep_riverwater_motion"
 	immerse_overlay = "immerse_deep"
-	slowdown = 8
 	baseturfs = /turf/open/water/no_planet_atmos/deep
 
 /turf/open/water/no_planet_atmos/deep/make_immersed()
@@ -107,7 +106,6 @@
 /turf/open/water/deep_beach
 	name = "deep water"
 	desc = "Don't forget your life jacket."
-	slowdown = 8
 	immerse_overlay = "immerse_deep"
 	icon = 'icons/turf/beach.dmi'
 	icon_state = "deepwater"

code/modules/movespeed/modifiers/mobs.dm

@@ -179,10 +179,15 @@
 /datum/movespeed_modifier/magic_ties
 	multiplicative_slowdown = 0.5
 
-///speed bonus given by the fish tail organ when inside water.
+///Speed bonus given by the fish tail organ when inside water.
 /datum/movespeed_modifier/fish_on_water
 	blacklisted_movetypes = MOVETYPES_NOT_TOUCHING_GROUND
 	multiplicative_slowdown = - /turf/open/water::slowdown
 
+///Slowdown for swimming on deep water tiles
+/datum/movespeed_modifier/swimming_deep
+	blacklisted_movetypes = MOVETYPES_NOT_TOUCHING_GROUND
+	multiplicative_slowdown = 7
+
 /datum/movespeed_modifier/tail_dragger
 	multiplicative_slowdown = 4