Use whatever you used to fix the aquapanel. If the gap isn't deep then build it up with bonding or tile adhesive. The tiles however, should be fixed with tile adhesive.
I suppose they have a point about corrosion and 'popping'. Chances of it happening in real life though? Stainless would do same job.
Bit of a cash cow. Another one that gets me is those green coated coach bolts for exterior use. They cost silly money when a normal galvanised bolt will outlast the timber.
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