In act_move.c, function move_char( ), locate this section:

	    if ( (ch->mount && !IS_FLOATING(ch->mount)) || !IS_FLOATING(ch) )
	    {
		/*
		 * Look for a boat.
		 * We can use the boat obj for a more detailed description.
		 */

Suppose ch has a flying/floating mount, but ch themselves are not flying/floating. If so, why the hell would you still be checking for a boat? That's kinda... not right. There's also no accounting for being able to breath water.

That should probably read more like this:

	    if ( ( !ch->mount && !IS_FLOATING( ch ) && !IS_AFFECTED( ch, AFF_AQUA_BREATH ) )  
	      || ( ch->mount && !IS_FLOATING( ch->mount ) && !IS_AFFECTED( ch->mount, AFF_AQUA_BREATH ) ) )
	    {
    		/*
    		 * Look for a boat.
    		 * We can use the boat obj for a more detailed description.
    		 */