Sorry to hear that your pet is vomiting. In your post you stated that your pet vomits up mucousy-watery fluid 5-10 times per day. I am assuming this is not always immediately after drinking?
Make an appointment with your veterinarian. A physical examination, discussing the history and and routine senior dog blood/urine testing may answer a lot of questions for you. Has yearly blood or urine tests been done?
There can be a long list of causes. Some are simple and not a big deal. Stomach upset can be an early sign of problems because it is a large surface of a mucous membrane. As such it is blood rich, and any problems that might be altering blood chemistry may show up as stomach upset/vomiting.
You may want to ask your vet if Pepcid (famotidine) could be helpful. I will often use 20mg of famotidine by mouth every 24 hours for a 50-60 pound pooch to help with upset tummies! (Ask your vet first!)
Congestive heart failure (CHF) ?
In general the most common sign will be coughing which may lead to a pet even coughing up fluid and foam. Typically CHF will be more coughing than vomiting. A physical examination by a veterinarian can often determine if CHF is of a concern.