avoid "<mon> slithers under the water" for fish

Don't use "slither" for movement action when observing an aquatic
monster go into hiding underwater.  Use "dive" instead.

Shark, pirahna, and jellyfish had been flagged M1_SLITHY but aren't
anymore.  Giant eel and electric eel are still M1_SLITHY and kraken
wasn't and still isn't.

There may be some odd cases that used to use slither and it went by
unnoticed where now use of the default verb might become noticeable.

include/monsters.h

@@ -1,4 +1,4 @@
-/* NetHack 3.7	monsters.h	$NHDT-Date: 1665130023 2022/10/07 08:07:03 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.103 $ */
+/* NetHack 3.7	monsters.h	$NHDT-Date: 1689793237 2023/07/19 19:00:37 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.109 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /*-Copyright (c) Michael Allison, 2006. */
 /* NetHack may be freely redistributed.  See license for details. */
@@ -2713,26 +2713,33 @@
         M3_INFRAVISIBLE, 8, CLR_YELLOW, DJINNI),
     /*
      * sea monsters
+     *
+     * 3.7: all the fish except kraken used to specify M1_SLITHY, presumably
+     * cloned from giant eel.  Using "slither" to describe their movmement
+     * wasn't appropriate.  Unfortunately, locomotion() isn't able to choose
+     * "swim" as their movement description because it is only passed a
+     * monster type, not a specific monster (for <mx,my>) or the relevant
+     * location, and therefore doesn't know whether water is involved.
      */
     MON("jellyfish", S_EEL, LVL(3, 3, 6, 0, 0), (G_GENO | G_NOGEN),
         A(ATTK(AT_STNG, AD_DRST, 3, 3), NO_ATTK, NO_ATTK, NO_ATTK, NO_ATTK,
           NO_ATTK),
         SIZ(80, 20, MS_SILENT, MZ_SMALL), MR_POISON, MR_POISON,
-        M1_SWIM | M1_AMPHIBIOUS | M1_SLITHY | M1_NOLIMBS | M1_NOHEAD
+        M1_SWIM | M1_AMPHIBIOUS | M1_NOLIMBS | M1_NOHEAD
             | M1_NOTAKE | M1_POIS,
         M2_HOSTILE, 0, 5, CLR_BLUE, JELLYFISH),
     MON("piranha", S_EEL, LVL(5, 18, 4, 0, 0), (G_GENO | G_NOGEN | G_SGROUP),
         A(ATTK(AT_BITE, AD_PHYS, 2, 6), ATTK(AT_BITE, AD_PHYS, 2, 6),
           NO_ATTK, NO_ATTK, NO_ATTK, NO_ATTK),
         SIZ(60, 30, MS_SILENT, MZ_SMALL), 0, 0,
-        M1_SWIM | M1_AMPHIBIOUS | M1_ANIMAL | M1_SLITHY | M1_NOLIMBS
+        M1_SWIM | M1_AMPHIBIOUS | M1_ANIMAL | M1_NOLIMBS
             | M1_CARNIVORE | M1_OVIPAROUS | M1_NOTAKE,
         M2_HOSTILE, 0, 7, CLR_RED, PIRANHA),
     MON("shark", S_EEL, LVL(7, 12, 2, 0, 0), (G_GENO | G_NOGEN),
         A(ATTK(AT_BITE, AD_PHYS, 5, 6), NO_ATTK, NO_ATTK, NO_ATTK, NO_ATTK,
           NO_ATTK),
         SIZ(500, 350, MS_SILENT, MZ_LARGE), 0, 0,
-        M1_SWIM | M1_AMPHIBIOUS | M1_ANIMAL | M1_SLITHY | M1_NOLIMBS
+        M1_SWIM | M1_AMPHIBIOUS | M1_ANIMAL | M1_NOLIMBS
             | M1_CARNIVORE | M1_OVIPAROUS | M1_THICK_HIDE | M1_NOTAKE,
         M2_HOSTILE, 0, 9, CLR_GRAY, SHARK),
     MON("giant eel", S_EEL, LVL(5, 9, -1, 0, 0), (G_GENO | G_NOGEN),

src/mon.c

@@ -4259,7 +4259,8 @@ hideunder(struct monst *mtmp)
 {
     struct trap *t;
     struct obj *otmp;
-    const char *seenmon = (char *) 0, *seenobj = (char *) 0;
+    const char *seenmon = (char *) 0, *seenobj = (char *) 0,
+               *locomo = (char *) 0;
     int seeit = gi.in_mklev ? 0 : canseemon(mtmp);
     boolean oldundetctd, undetected = FALSE, is_u = (mtmp == &gy.youmonst);
     coordxy x = is_u ? u.ux : mtmp->mx, y = is_u ? u.uy : mtmp->my;
@@ -4276,8 +4277,10 @@ hideunder(struct monst *mtmp)
            under water unless some obstacle blocks line-of-sight */
         undetected = (is_pool(x, y) && !Is_waterlevel(&u.uz)
                       && (!Underwater || !couldsee(x, y)));
-        if (seeit)
+        if (seeit) {
             seenobj = "the water";
+            locomo = "dive";
+        }
     } else if (hides_under(mtmp->data)
                /* hider-underers only hide under objects */
                && (otmp = gl.level.objects[x][y]) != 0
@@ -4314,8 +4317,9 @@ hideunder(struct monst *mtmp)
            level creation because 'seeit' will be 0 so 'seenmon' and 'seenobj'
            will be Null */
         if (undetected && seenmon && seenobj) {
-            You_see("%s %s under %s.", seenmon,
+            if (!locomo)
-                    locomotion(mtmp->data, "hide"), seenobj);
+                locomo = locomotion(mtmp->data, "hide");
+            You_see("%s %s under %s.", seenmon, locomo, seenobj);
             iflags.last_msg = PLNMSG_HIDE_UNDER;
             gl.last_hider = mtmp->m_id;
         }

src/mondata.c

@@ -1283,6 +1283,10 @@ static locoverbs levitate = { "float", "Float", "wobble", "Wobble" },
                  flys = { "fly", "Fly", "flutter", "Flutter" },
                  flyl = { "fly", "Fly", "stagger", "Stagger" },
                  slither = { "slither", "Slither", "falter", "Falter" },
+                 /* it would be useful to incorporate "swim" but we lack
+                  * sufficient information to know whether water is involved
+                 swim = { "swim", "Swim", "flop", "Flop" },
+                  */
                  ooze = { "ooze", "Ooze", "tremble", "Tremble" },
                  immobile = { "wiggle", "Wiggle", "pulsate", "Pulsate" },
                  crawl = { "crawl", "Crawl", "falter", "Falter" };