We tied up there briefly on a jenneau 40 which had 1.8m draft if that helps.

Our boat is deep too (not the jenneau)! Can't really remember what was under us when tied up but it was a training school boat while day skipper training so it was likely plenty... Just not 100% sure what it was now.

If I remember correctly though, I think you can anchor just outside the entrance to the Raby bay channel entrance and perhaps jump in the tender.

The public pontoon is a busy location used by many charter boats. So you won't be popular if you stay there for any length of time

I suggest you anchor in the large basin just before the Marina and use the tender to go to the pontoon. You can tie up on the inside of the pontoon out of the way of other users.
Or you drop crew off on the pontoon and then come back to collect after shopping.

There is over 3m of water in the basin and the entrance channel.