Hi Rashni,
I live north of Townsville and paddle in the same hot water this time of year.Probably more your feet getting used to stand up. Mine used to ache when I first started,feet,shins,knees all used to give me crap but after a while not a problem dont even think about the now.Try wiggling the toes and moving on your board a bit, take breaks and put your feet up.After awhile you wont even think about them.

as others have said, most likely from toe gripping / toe clawing - very common in those new to SUP or if you've had a break from paddling. A few things that can help:Calf muscle stretches (particularly the version with your toes up on the wall and your heel on the ground)
try some single leg balance exercises on land - eyes open and eyes closed, but bend your toes up the opposite way (towards your shin) aim for 30s of stable balancing with your toes up, not curled
Get a tennis ball or similar, whilst sitting down watching TV, roll the tennis ball along the sole / arch of your foot for a massage