Initially it will increase because of vasoconstriction- your body constricts surface blood vessels to conserve heat for your vital organs. Blood pressure and heart rate
increase whilst your muscles tense and shiver which generates more body heat, if the water gets colder you will then find that your HR and BP etc will actually decrease.
Immersion into cold water increase the blood flow returning to the heart, causing a reflexive slowing of heart rate. Just make sure the water you jump into isn't too cold.