CDX grade plywood is fine for outdoor use, the glue is waterproof so the plywood will not delaminate over time, you can expect it to last at least 25 years. I would put a preservative coating on all surfaces prior to the tile cement, paying added attention to the cut edges.
You may consider not using plywood at all. Once you frame the table, user cement backer board with a decoupling layer since those are the preferred substrates for tile.
Then you will have much fewer concerns about moisture, because the tile will be properly bonded, and provide its own waterproofing layer over the wood framing.
While t+g might have been a bit better, as long as your sheets landed well (1/2 on, 1/2 off) the joists, it'll be fine.
If you're still concerned AND you can get underneath, you could install solid blocking (2x4 on the flat) perpendicular to the joists under the seams.
Edit to reflect new information (seams not on joists):
Soooo... this isn't great, but ...