Hv, we've had some very hot an humid weather the last few days, and I have noticed that my blood pressure has been very low. I do drink a lot of water when it is hot, but add a little lake salt to it, because I am aware that drinking a lot of water can put your electrolyte balance out of whack.

Yes, drinking too much water too quick can cause the electrolytes to go out of whack and swell the brain.

Do you think that my potassium levels could be too low, hence the low blood pressure, (I took my blood pressure reading because I was getting dizzy spells when I stood up from sitting or laying down), or some other mineral imbalance?

Getting dizzy when standing up is normally from orthostatic hypotension. This is caused from the blood pressure dropping when you stand up. Gravity pulls blood away from the brain temporarily leading to the dizziness. Adrenal dysfunction plays a primary role here. When the blood pressure drops from gravity's pull the adrenals should kick out epinephrine (adrenaline) in response to constrict the blood vessels to maintain blood flow to the brain. When the adrenals are slow to react we get short dizzy spells upon standing until the adrenals kick in and kick out that epinephrine. 

Heat also dilates our blood vessels further contributing to the drops in blood pressure. Consider how we can feel dizzy when we are in a hot spa or sauna too long. This is not from electrolyte loss but rather vasodilation.

If you are prone to chronically low blood pressure to begin with this can indicate low adrenal function to begin with and can further contribute to the problem since the blood pressure is already low to begin with.

Potassium actually lowers the blood pressure by displacing sodium. So I do not recommend increasing your potassium. Instead taking licorice root would be better. The licorice root helps you to retain sodium and will help strengthen the adrenals so they respond to the blood pressure drop properly.

Supporting the adrenals would be a good idea regardless since the adrenals help to regulate electrolyte balance.