To get enough power to heat at flow of water you'll need almost a commercial system if you go tankless. You almost certainly don't have a big enough gas line, and would likely pay a demand fee that is significantly higher than what you have now, even though you wouldn't be using it all that often. You might need a gas line upgrade a couple of sizes larger.
A WH is normally good for about 70% of its volume from a high flow situation, which filling the tub would be. With a tank, you can store the heat rather than dumping it all in at once while there is demand.
A WH doesn't lose huge amounts of energy, especially if it is in an area where you would be heating anyways. In MA, your heating season is much longer than your cooling one.
In MA, it is likely that the incoming water temp in the winter could approach freezing on a long cold spell. A tankless, especially at the flows you are talking about would need a huge amount of heat. A gallon is about 8#, so at 17gpm, that's 136# of water per minute. Say it needs to raise that water from 33 to 110, that's 10,472BTU/minute or 628,320BTU/hour. If you assume maybe 85% efficiency, your input energy would be 739,200BTU/hr. Increase the flow by someone else running a shower or the washing machine, or dishwasher and you could be looking at a one MILLION BTU system. Most residential units peak around 200K, so that'd be 5. You could probably by a nice couple of cars for the kids for what it would cost to install it, if you had room.