Don't Hail A Cab
While Manta is significantly smaller than Quito or some of the other big cities in Ecuador I would not hail a cab from a street corner. If you need a cab call a radio controlled cab company from your hotel or pension. Be sure and note the cab company and name of the driver of the cab. While Manta is generally safe there have been reports of kidnapping and hold ups from cab companies in the area. Better to be safe than sorry.
Hard Hat Area
The commercial pier where the cruise ships dock is not for tourists. It is a dangerous commercial area with cranes and heavy equipment. It isn't for walking.
In addition during the time we were there, an ammonia refrigeration plant on one of the tuna boats exploded, killing several people and causing injuries to others. The port was closed for a period of time for emergency vehicle access.
not safe in non tourist areas
You have to be careful at night. don't want to scare nobody but do not walk alone in the streets. groups are good. don't wear flashy jewlery it will be snatched. try to stay in tourist areas. be safe when venturing in unknown areas. take a taxi it is safe and they only charge like 1 or two.