Can i use a water heater as my source for the in floor Pex tubing system?
It's all about heat load.

A very well insulated building may have a heat load so low that most boilers would be operating very inefficently to meet the load - sitting idle most of the time, even on "design days" when the heat load is at the greatest due to the outside temperature being the lowest temperature designed for (thus, "design day.")

As a practical example, I have a radiant floor system installed in a building with a design-day load of 30-36,000 BTU/hr. A typical "small" boiler is 100,000 BTU/hr, so it would be in standby 2/3 of the time even on that coldest day (which is typically for no more than a week, and not every year.) A water heater can be more closely matched to the load, simply because they are available in smaller sizes. 

As for tank .vs. no tank, it's largely irrelevant - there are inefficient and efficient versions of both (and much of the hype for tankless heaters being "more efficient" due to no standby losses falls down in houses where hot water is actually used, at modern insulation levels which lower standby losses; especially considering greater complexity and maintenance expense that is often associated with tankless.) A common example (and commonly used for radiant heat) of an efficient tank type is the polaris, at something like 95% efficient and a condensing design. 

If you have a larger heat load, an efficient condensing boiler might be a better choice - that has to to with the whole building construction, not the heat delivery method.