Set Instant Pot to sauté. Melt butter then add arborio rice. Toast the rice for about 3 minutes, until the rice becomes translucent and the edges become golden.
Whisk together whole milk, water, and sugar and add to the Instant Pot.
Set pot to manual and adjust to high pressure for 10 minutes making, sure the valve is in the sealing position. Once it is finished, allow the pressure to release naturally for 10 minutes and then do a quick release. This will prevent the milk from bubbling through the valve.
While the pressure is releasing whisk together eggs, half and half, and vanilla.
Once the pot has been opened, add 1/2 cup of the hot rice into the egg mixture, whisking constantly.
Return back to the Instant Pot with the optional cinnamon and raisins, and set to sauté, continually stirring for 3 minutes.
Serve either warm or cold. Be sure to press plastic wrap onto the rice pudding if refrigerating, to prevent a film.