No, deposits on Splash cannot be used to pay for Private (Self-Managed) contests.
Why Not?
Splash does not handle entry fees or payouts for Self-Managed contests — see Do Members Need to Pay for a Private (Self-Managed) Contest? for the full breakdown
Commissioners are responsible for collecting and distributing payments externally
Splash Credits also cannot be used toward self-managed contest payments
What If I Want to Use My Splash Balance?
Deposits and credits on Splash are only usable for Splash-Managed contests, where entry fees and payouts happen within the platform. If you're running a self-managed contest, you'll need to coordinate payments directly with your members instead.