some fish should lay their eggs in the water rather than on land

Generally, fish should lay their eggs in the water and
not on land, but the game was only allowing the opposite.

Eels are catadromous and lay their eggs in the Sargasso Sea,
not in the dungeon.

include/mondata.h

@@ -71,6 +71,8 @@
 #define slimeproof(ptr) \
     ((ptr) == &mons[PM_GREEN_SLIME] || flaming(ptr) || noncorporeal(ptr))
 #define lays_eggs(ptr) (((ptr)->mflags1 & M1_OVIPAROUS) != 0L)
+#define eggs_in_water(ptr) \
+    (lays_eggs(ptr) && (ptr)->mlet == S_EEL && is_swimmer(ptr))
 #define regenerates(ptr) (((ptr)->mflags1 & M1_REGEN) != 0L)
 #define perceives(ptr) (((ptr)->mflags1 & M1_SEE_INVIS) != 0L)
 #define can_teleport(ptr) (((ptr)->mflags1 & M1_TPORT) != 0L)