Though I believe the other answer is right, in that a planet as small as you're suggesting would be unable to keep an atmosphere (and therefore an ocean) due to lack of gravity, there are ways for a smaller planet to maintain a liquid ocean that supports life:
"Owing to a greater depth of hydrothermal circulation from thermal cracking in brittle mantle material, small ocean planets in the Solar System may have the capacity to support ecosystems that are stable on geologic timescales, with greater amounts of bioavailable energy than previously suggested. " http://online.liebertpub.com/doi/pdf/10.1089/ast.2007.0075
Assuming there is some sort of force keeping the water on the surface (not to mention the atmosphere), you'd be dealing with several things:
1. Reduced water pressure - If you've got 1/2 moon gravity, that's about 8% earth gravity, so the water pressure would be reduced as the weight of the water would be less. I'd guess that would mean that creatures would either more easily traverse greater depth ranges or be confined to smaller ranges of pressure than earth creatures. Interestingly, this doesn't mean less dense water, as water is essentially incompressible, and the difference in ocean water density has more to do with temperature and salinity than pressure.
2. Lack of light - There's a possibility of oceans existing underground or beneath ice on smaller planets, so organisms would likely have to exist either without or with very limited exposure to sunlight. In that case, this probably limits the food chain to very small, efficient organisms. 258
Bacteria, etc. See the above-quoted article for more information on the placement of oceans.
3. Ice - If your ocean is too deep, the high pressure would turn the water to ice. So watch for that.
4. Ocean coverage - If the ocean covers the entire planet, some of the precursors to earth-like life would be difficult to come by. On earth, phosphorous is washed by rainwater into oceans from rocks on exposed land, and that's a necessary element for the development of plankton and similar life, so depending on your planet, those might not exist.
"It turns out that water worlds may be some of the worst places to look for living things. One study presented at the meeting shows how a planet covered in oceans could be starved of phosphorus, a nutrient without which earthly life cannot thrive. Other work concludes that a planet swamped in even deeper water would be geologically dead, lacking any of the planetary processes that nurture life on Earth." https://www.nature.com/news/exoplanet-hunters-rethink-search-for-alien-life-1.23023 279