[Varyon]

Looking at Spaceships, if you want around 12 G's with water as the reaction mass, you're looking at superscience. TL9+ Nuclear Thermal Rockets do come close, however - using water as the reaction mass, they've got 1.5 G each, for a maximum of 9 G with 6 systems (total 3/10ths of the vessel's mass). With 10 fuel tanks of water (half the vessel's mass), you'll get 2.1 mps delta-V which isn't going to get you much of anywhere fast. You can arbitrarily multiply the delta-V by whatever factor you'd like for your cinematic rockets, of course. Another option might be to allow for high-thrust, like the various fusion reaction engines have access to - this is double acceleration, half delta-V, or 3 G per system (18 G max) and a total of 1.05 mps delta-V when using water; again, you'll need to markedly increase delta-V for this to work out.

If you're willing to go to some limited superscience, the TL 10^ Fusion Torch is an option. Using water, and again with half the vessel's mass in fuel, you're looking at 1.5 G per system and 70 mps delta-V at TL 10^, 210 mps delta-V at TL 11^, and 700 mps delta-V at TL 12^. High-thrust is canonically an option here, for double acceleration (3 G per system) and half delta-V (35, 105, 350 mps, respectively). As an example of what you can manage, let's go with the worst-case - high-thrust water at TL 9^. To go from Earth orbit to Mars orbit, first you've got to burn 2.1 mps to break orbit. You've got 33.9 mps left to play with, but you need to account for both acceleration and deceleration. Deceleration is 0.93 mps less than acceleration (you just need to slow down to orbital velocity, not to a dead stop). If we use all but 0.93 mps delta-V (leaving a little room for error, and making the math easier), that means we accelerate to 16.95 mps, cruise for a while, then burn another 16.02 mps to slow down to orbital velocity. If we accelerate at a comfortable 1 G (we only need the high acceleration for combat), this takes around 45 minutes (12 G would take around 4 minutes), during which time we travel 0.00025 AU, which is pretty much a rounding error compared to the 1.5 AU between Earth and Mars, so we'll ignore it for the next calculation. Traveling 1.5 AU at 16.95 mps takes a bit over 95 days (95 days, 5 hours, 16 minutes, and 48 seconds), then another ~45 minutes to slow down to 0.93 mps. You're probably going to want higher delta-V. One option might be to have an adjustable drive - it only suffers the halved delta-V when used in high-thrust mode (such as in combat). I'll leave it to you to work out the math on that and the other delta-V's (although IIRC, Pyramid #3/79 comes with a spreadsheet that can do most of the work for you).