Pallet wood is cheap, soft wood. Unless you can put a preservative on them, they will not last. Not worth the time and energy using pallet wood around anything that has todo with water.
Here are some ideas for a short-term DIY:
Here are the one I made. I thought they were pretty cool.
I made a planter box 1st with cinder blocks,then cut old pallet pieces measured from the top with 2 extra inches that gets hammered into the ground...the top cinder cap will help keep the slats in piece