Improve the solo sub's water culling volumes. Problems were caused by the water culling shader having a global maximum of four at a time. The solo sub needed six on its own. Probably not a good idea to have that many anyway, so rather than change the shader, reduced it to four per sub, and the interior mesh will just need a minor edit to not show any water inside. The culling volumes are sorted so having e.g. two subs near each other is fine.