This is a tough question to answer in a simple way. Like you mentioned, there are several factors to take into consideration.
Given that CO=HR x SV, where HR is heart rate, CO is cardiac output, and SV is Stroke Volume, you can use this equation to help conceptualize. Keep in mind, HR is a rate/frequency and contractility is FORCE of contraction or how strong or weak that contraction is/can be.
First and foremost, warmth causes vasodilation, meaning vessels get bigger which causes an increased flow but decreased pressure. Theoretically we could use the warmth of the water to discuss this, but the heart and body will still respond to it as if its any given fluid and behave accordingly.
If we are talking about the entire cardiovascular system, it becomes easier to discuss. By adding fluid, forget warm or cold, you increase preload which increases SV and thereby contractility. The increased SV is due to added volume, and since the heart is receiving more blood, it will slow down (HR) so as to give the ventricles time to fill. This increase in SV causes the HR to decrease in order to maintain the CO. It can also cause an increase in cardiac output as the force of contraction and SV are increased, but that is a little tough to type out. The body typically tries to stay in homeostasis and "likes" to maintain the same CO regardless of changes happening with HR or SV. When talking about hypovolemic shock and volume losses, it becomes easier to discuss. Nonetheless, let's arbitrarily assign values to the equation:
If CO=HRxSV, where CO=100, HR=25, and SV=4, we see that 100=25x4.
If SV increases to let's say 5, in order to maintain the same cardiac output of 100, HR HAS to decrease to 20.
I can go more in depth on this and properly describe it using my iPad and pencil. It's easier to understand that way