In Overworld_FieldMoves

EventHandlers.add(:on_player_interact, :start_surfing,
  proc {
    next if $PokemonGlobal.surfing
    next if $game_map.metadata&.always_bicycle
    next if [436, 438].include?($game_map.map_id) #Navel Rock map ids
    next if !$game_player.pbFacingTerrainTag.can_surf_freely
    next if !$game_map.passable?($game_player.x, $game_player.y, $game_player.direction, $game_player)
    pbSurf
  }
)


Under HiddenMoveHandlers::CanUseMove.add(:SURF, proc { |move, pkmn, showmsg|
  if [436, 438].include?($game_map.map_id) #Navel Rock map ids
    pbMessage(_INTL("You can't surf here.")) if showmsg
    next false
  end