diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
index b720aac57d..f28306c2a3 100644
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -459,6 +459,13 @@ struct GenerateLandscapeWindow : public QueryStringBaseWindow {
 		this->SetWidgetDisabledState(GLAND_START_DATE_UP,   _settings_newgame.game_creation.starting_year >= MAX_YEAR);
 		this->SetWidgetDisabledState(GLAND_SNOW_LEVEL_DOWN, _settings_newgame.game_creation.snow_line_height <= MIN_SNOWLINE_HEIGHT || _settings_newgame.game_creation.landscape != LT_ARCTIC);
 		this->SetWidgetDisabledState(GLAND_SNOW_LEVEL_UP,   _settings_newgame.game_creation.snow_line_height >= MAX_SNOWLINE_HEIGHT || _settings_newgame.game_creation.landscape != LT_ARCTIC);
+
+		/* Do not allow a custom sea level with the original land generator. */
+		if (_settings_newgame.game_creation.land_generator == 0 &&
+				_settings_newgame.difficulty.quantity_sea_lakes == CUSTOM_SEA_LEVEL_NUMBER_DIFFICULTY) {
+			_settings_newgame.difficulty.quantity_sea_lakes = CUSTOM_SEA_LEVEL_MIN_PERCENTAGE;
+		}
+
 	}
 
 	virtual void UpdateWidgetSize(int widget, Dimension *size, const Dimension &padding, Dimension *fill, Dimension *resize)
@@ -658,9 +665,15 @@ struct GenerateLandscapeWindow : public QueryStringBaseWindow {
 				ShowDropDownMenu(this, _elevations, _settings_newgame.difficulty.terrain_type, GLAND_TERRAIN_PULLDOWN, 0, 0);
 				break;
 
-			case GLAND_WATER_PULLDOWN: // Water quantity
-				ShowDropDownMenu(this, _sea_lakes, _settings_newgame.difficulty.quantity_sea_lakes, GLAND_WATER_PULLDOWN, 0, 0);
+			case GLAND_WATER_PULLDOWN: { // Water quantity
+				uint32 hidden_mask = 0;
+				/* Disable custom water level when the original map generator is active. */
+				if (_settings_newgame.game_creation.land_generator == 0) {
+					SetBit(hidden_mask, CUSTOM_SEA_LEVEL_NUMBER_DIFFICULTY);
+				}
+				ShowDropDownMenu(this, _sea_lakes, _settings_newgame.difficulty.quantity_sea_lakes, GLAND_WATER_PULLDOWN, 0, hidden_mask);
 				break;
+			}
 
 			case GLAND_SMOOTHNESS_PULLDOWN: // Map smoothness
 				ShowDropDownMenu(this, _smoothness, _settings_newgame.game_creation.tgen_smoothness, GLAND_SMOOTHNESS_PULLDOWN, 0, 0);
diff --git a/src/landscape.cpp b/src/landscape.cpp
index db54810b23..71a788c25c 100644
--- a/src/landscape.cpp
+++ b/src/landscape.cpp
@@ -963,6 +963,7 @@ void GenerateLandscape(byte mode)
 			default: {
 				uint32 r = Random();
 
+				assert(_settings_game.difficulty.quantity_sea_lakes != CUSTOM_SEA_LEVEL_NUMBER_DIFFICULTY);
 				uint i = ScaleByMapSize(GB(r, 0, 7) + (3 - _settings_game.difficulty.quantity_sea_lakes) * 256 + 100);
 				for (; i != 0; --i) {
 					GenerateTerrain(_settings_game.difficulty.terrain_type, 0);
diff --git a/src/settings.cpp b/src/settings.cpp
index e91f1cf69d..7ef204045b 100644
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -902,6 +902,21 @@ static bool InvalidateIndustryViewWindow(int32 p1)
 	return true;
 }
 
+/** Checks if any settings are set to incorrect values, and sets them to correct values in that case. */
+static void ValidateSettings()
+{
+	/* Force the difficulty levels to correct values if they are invalid. */
+	if (_settings_newgame.difficulty.diff_level != 3) {
+		SetDifficultyLevel(_settings_newgame.difficulty.diff_level, &_settings_newgame.difficulty);
+	}
+
+	/* Do not allow a custom sea level with the original land generator. */
+	if (_settings_newgame.game_creation.land_generator == 0 &&
+			_settings_newgame.difficulty.quantity_sea_lakes == CUSTOM_SEA_LEVEL_NUMBER_DIFFICULTY) {
+		_settings_newgame.difficulty.quantity_sea_lakes = CUSTOM_SEA_LEVEL_MIN_PERCENTAGE;
+	}
+}
+
 /*
  * A: competitors
  * B: competitor start time. Deprecated since savegame version 110.
@@ -941,17 +956,6 @@ void SetDifficultyLevel(int mode, DifficultySettings *gm_opt)
 	}
 }
 
-/**
- * Checks the difficulty levels read from the configuration and
- * forces them to be correct when invalid.
- */
-static void CheckDifficultyLevels()
-{
-	if (_settings_newgame.difficulty.diff_level != 3) {
-		SetDifficultyLevel(_settings_newgame.difficulty.diff_level, &_settings_newgame.difficulty);
-	}
-}
-
 static bool DifficultyReset(int32 level)
 {
 	SetDifficultyLevel(level, (_game_mode == GM_MENU) ? &_settings_newgame.difficulty : &_settings_game.difficulty);
@@ -1477,7 +1481,7 @@ void LoadFromConfig()
 	IniLoadSettings(ini, _gameopt_settings, "gameopt", &_settings_newgame);
 	HandleOldDiffCustom(false);
 
-	CheckDifficultyLevels();
+	ValidateSettings();
 	delete ini;
 }