I did not feel Santa Marta was exremely great. Although, we were there for only 1 day and night. The walk from the hotel to the bus station was very sketchy and felt dangerous.
The beach is OK but also shares it with a large industrial marine port. However if you are moving on to Taganga, or Tayrona, a night in SM is worth seeing.
We visited tbe Tayrona national park which is an absolute must. One day in the park was more than enough for us but some people stay there camping.
We also went to tbe Crystal beach by boat which is gorgeous and great place to stay. Dont rent a mask as the sealife is not that rich.