A few things that you need to consider when using a plate liquid to liquid heat exchanger. One thing is that the BTUs that it is rated to adsorb are based upon several factors, the first of which is the differential temperature between the two fluids, obviously the greater the differential temp the more heat is transferred that is of course if you have the rated amount of fluid flow going through both sides of the heat exchanger. If you are not moving the fluid through the exchanger at a rate that is fast enough to ensure that the flow is turbulent then your heat exchange efficiency will suffer. Actually the faster the water flows the better. This brings up an additional potential problem. If the exchanger is rated at say 20 gpm flow don't think that you can just attach a 20 gpm nominally rated pump and have 20 gpm. As most if not all water pumps are centrifugal their flow rate drops as the out put pressure is increased, by such things as pressure drop across your plate heat exchanger so you will probably need a much larger pump just to ensure that you are getting enough water going through the exchanger, and that goes for both the engine cooling water and the water from your tank. Neil is correct that the act of bringing water to boiling does require additional heat, the heat of transition, but if you are going to boil your tank water away that would require that your engine water be higher than boiling, not a good idea. I would much sooner go with your concept of a closed tank system with lots of ice,which also requires additional heat to transition from ice to water and properly sized water pumps to insure good flow for both the cooling water and the engine water. I have applied plate heat exchangers a number of times and they can be very efficient and are of course very appealing because of their small size. A liquid to liquid i.e. water to water in this case if properly designed, proper flow rates through both sides of the exchanger will typically be 7-8 times more efficient that an air to water exchanger, also assuming that its water flow rate and the rate of the air passing through it are correctly sized.
Rex