One important note (if you haven't done it already). Build a fortification in the tunnel that leads to the reservoir. This should prevent any aquatic-based !!fun!! from entering through it (like some titans and occasionally some invader mounts).
Actually once a fortification is 7/7 in liquids then anything that can swim can go through it just fine.
To protect against everything there is really only one reliable way that protects against 100% of all problems from getting in.
X = wall
≈ = water
W = well
# = grate
Side view
Prior to water flow
X≈≈≈XXW
XX XXX X
XX XXX#X
XX X
XXXXXXXX
After water flow
X≈≈≈XXW
XX≈XXX≈X
XX≈XXX#X
XX≈≈≈≈≈X
XXXXXXXX
Because building destroyers cannot destroy things above them, the only true safe water source is to use pressure to drive the water up through a floor grate with pressure and then access it there.
And on the topic of pressure, there are a few simple rules.
0)Magma never rises unless driven through a pump, when it will follow Rule #1.
1)Water driven through pumps will attempt to rise to the same level as the pump outlet.
2)Damned rivers function identical to pump driven water on their level.
3)Water from water sources will attempt to rise to the same level as the source.
3b)If sourced water goes through a u-bend (as seen in the design above) it will attempt to rise to 1 z-level below the source (as opposed to the source level).
4)If water goes through a diagonal connection then it is treated as sourced water with the source on the same level as the diagonal connection. This will transform pumped water into sourced water.