diff --git a/include/monsters.h b/include/monsters.h
index 6e3a2093e..be0f17557 100644
--- a/include/monsters.h
+++ b/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),
diff --git a/src/mon.c b/src/mon.c
index 201092d90..40e296756 100644
--- a/src/mon.c
+++ b/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,
-                    locomotion(mtmp->data, "hide"), seenobj);
+            if (!locomo)
+                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;
         }
diff --git a/src/mondata.c b/src/mondata.c
index d2440b548..312920c75 100644
--- a/src/mondata.c
+++ b/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" };