It's not written how deep the rocks are, or how much watter were above them in Jesus' time, but just imagine not much and that he knew where they were -- then the scenes where he gets out of the boat to walk on the watter get a very obvious explanation.
Yes, surely the fishermen who were sailing it would be ignorant of shallow rocks right next to the boat. It's not like they would have to avoid hitting them.
The rates are the same ($1 per million writes and $0.70 per million reads beyond the daily free threshold), but the daily free threshold is 0.05 million of each for master/slave, and 0.01 million of each for high-replication.
For small applications it costs more because of thresholds for free services is lower. In our case it costs a lot more since some of things we were doing need another instance on HRD that we didn't need on M/S