HGH/mk677 will cause significant water retention the first 1-2 weeks of using it. As your body gets used to the compound, the water retention properties will subside significantly. After the first 2 weeks, you won't notice the water retention at all.
The good news - it means your mk677 is working and quality.
To reduce water retention, make sure you are drinking plenty of water throughout the day, and minimize your sodium intake (but don't eliminate it).