Is a 50/50 mix of Acetone and Water inert? We are cleaning a gel out of a fuel tank that is dissolved by Acetone.  We want to neutralize the acetone by adding water to achieve a mix of 50/50 so that (PERHAPS) the acetone will not damage rubber / plastic compounds in the fuel system. 

We have an electric fuel pump we are going to use to get the acetone and water out and we need to make sure we don't damage it. THEN there are the two brand new mercruiser marine fuel pumps, fuel filter and carburetor seals we don't want to damage.   We CAN  add more water than a 50/ mix as well. 

We know we can't get all the contents of the tank out and so we want to make sure any remaining acetone will not be of a concentration to ruin the fuel system components.  By the way... this is for a famous movie director Roman Coppola.
Acetone is notoriously aggressive towards the plastic, rubber, and other non-metal components of internal combustion engines (and associated parts like fuel pumps). If the concentration of acetone is strong enough to dissolve your jell (assuming that is some organic compound) it is very likely strong enough to attack components of the fuel pump.  

As a bit of anecdotal evidence regarding what I said above, note that of the dozen-odd oxygenated additives allowed by the US EPA for the oxygenated fuels program, neither acetone nor any other ketone is on the allowed list. Although this is in part due to the high volatility of acetone, it is also largely due to the fact that acetone would attack engine components as described above.  

In short, what you've described is a bad idea for the health of your fuel pumps, even at very low concentrations of acetone.