Did you replace the seal that goes between the waterpump and the case?
Did you use a torque wrench?
As said, I've rebuilt several Suzuki waterpumps, including my '03 Gsxr1k that I have be redoing, so the bike has been on and off my lift table upright in the wheel chock.
I rebuilt the waterpump, changed the oil, and went on to other projects on the bike.
Then I took that bike off of the lift to put another on it, and put my '03 on the kickstand.
Well, what you know, and oil leak...at the waterpump...yay.
How? I torqued the bolts to the service manual specs, and my Snap-On torque wrench was calibrated.
I then took a 1/4 drive ratchet and socket and snugged the bolts. Yes, they got tighter, and Yes...it stopped the oil leak.
Remember that those are steel bolts, going into aluminum, so it is very easy to strip or gual threads. I use a Small amount of anti-sieze to prevent that, but, an average of 10% less torque is adviseable first, then slowly add to it.
Sometimes you need to add a little more torque to certain fasteners, and they are usually small ones.
Proceed with caution when doing so, and if you don't have enough mechanical experience to know when a bolt has seated, and that the threads will strip if you continue, then you can learn to install a helicoil to fix it, lol
But seriously, try to slowly tighten the bolts, and see if they will turn anymore.
And, install the waterpump seal with a light coat of oil on it, which will help to prevent it from rolling, sticking, or tearing during installation.
You'll need to lean the bike to it's right side against something, or drain the oil to where there is none in the site glass(add it back when you're done obviously) before you remove the waterpump again, that way the oil level is below the opening in the case, and won't run out.
Make sure the impellar shaft is properly in the groove on the back of the waterpump, and engaged to the engine as well.