Players float when idle in water

This is done rather hackishly, but Minetest doesn't
seem to offer a sane way to register fluid buoyancy, or
even a reasonable hack to use negative gravity for this.
Instead, just push player upward with velocity impulses
every so often.

This is helpful gameplay-wise to make chatting while
swimming easier for players not using auto-forward, or
to prevent players from drowning and losing their stuff
while swimming long-distance and dropping network
connection and their in-game character waiting in the
water for the timeout.  Instead, players will return to the
surface and remain where they can breathe until the
player can take control again.

mods/nc_player_setup/buoyancy.lua

@@ -0,0 +1,36 @@
+-- LUALOCALS < ---------------------------------------------------------
+local minetest, pairs
+    = minetest, pairs
+-- LUALOCALS > ---------------------------------------------------------
+
+local liquids = {}
+minetest.after(0, function()
+		for k, v in pairs(minetest.registered_items) do
+			if v.liquidtype and v.liquidtype ~= "none" then
+				liquids[k] = true
+			end
+		end
+	end)
+
+local function checkbuoy(player)
+	if (not player.add_player_velocity)
+	or (player:get_player_control_bits() ~= 0) then return end
+
+	local pos = player:get_pos()
+	local node = minetest.get_node({
+			x = pos.x,
+			y = pos.y + (player:get_properties().eye_height - 1) / 2 + 1,
+			z = pos.z
+		})
+	if not liquids[node.name] then return end
+
+	return player:add_player_velocity({x = 0, y = 8, z = 0})
+end
+
+local function timer()
+	minetest.after(0.25, timer)
+	for _, player in pairs(minetest.get_connected_players()) do
+		checkbuoy(player)
+	end
+end
+minetest.after(0, timer)

mods/nc_player_setup/init.lua

@@ -8,3 +8,4 @@ nodecore.amcoremod()
 include("setup")
 include("hotpotato")
 include("globalstep")
+include("buoyancy")