I believe the story was in a book of 3 stories. The basic premise (if I remember correctly) is there are some individuals, 2 I think, researching on a planet. They are on some sort of hovering craft, and they discover that in the sea there is some type of large amorphous (perhaps glowing) organism that, when you immerse yourself in it, can commune with you and give higher thought. I believe in the end the main character decides to stay in the sea immersed in the organism, but I do not remember well enough to be sure (this was many years ago), but I am desperately looking for it. I do remember the other story in the book was about a man, who in the process of chasing an enemy, gradually begins to sacrifice his flesh for machine. The ultimate result is him turning into what is basically a mechanical dog, having sacrificed his humanity in order to destroy his enemy. It may have been called dog(s) or war or war dog, but I am also uncertain. I hope that is enough information. I cannot thank you all enough for the help.
You're thinking of Alastair Reynold's Revelation Space series. Specifically, Diamond Dogs, Turquoise Days.
Here's a brief description of the Pattern Jugglers, which match your sea organism:
The Pattern Jugglers are collectives of marine organisms that inhabit widely scattered water-covered planets. They act as a form of biotechnological data storage system, recording the minds and memories of anyone who communes with them. Nobody, including the jugglers themselves, knows their origin, although the inhibitors seem to avoid attacking Pattern Juggler worlds.
Pattern Jugglers record the neural structures of sentient beings who enter their oceans, and are capable of rewriting the brains of swimmers. Normally this involves giving them heightened abilities, such as expertise in mathematics. These abilities typically last only a short period of time, but sometimes they can last for the better part of a lifetime. It is possible to persuade the jugglers to perform desired modifications to the minds of human swimmers, normally by remembering special trigger symbols coded for a particular transformation.
You'll see Pattern Jugglers in Absolution Gap and Turquoise Days, for sure. What the summary above isn't clear on is that there many of these planets, all covered in water, and the water is all completely filled with them.
The character that stayed in the Pattern Jugglers was probably Naqi, in Turquoise Days:
The protagonist, Naqi, loses her sister at the start of the story when she swims with the Pattern Jugglers and is absorbed by them. Later on, a lighthugger, the Voice of Evening, visits the planet, the first lighthugger to do so in a long time. They're welcomed by the local government, and among the people on the lighthugger are a crew of scientists intending to study the Pattern Jugglers.
And at the end:
In the end, Naqi takes a final swim in the ocean, and joins her sister in the Juggler collective.
But some of this could also have been Antoinette Bax:
Antoinette opts to remain on the planet and is thus caught in the inhibitor bombardment of the planet (although it is implied that the Pattern Jugglers may have absorbed her into the ocean before the bombardment commenced).
Antoinette is one of the main characters Redemption Ark, but her story concludes in Absolution Gap.
Although other characters were also "absorbed" by this entity, such as Felka. Felka was a type of hyper-savant character that could perform extremely complex calculations in her head:
She eventually ends up on the colony on the planet Ararat and is 'taken' by the Pattern Jugglers there, presumably absorbed by them.
The person you're thinking of that slowly turned into machine is likely Richard from Diamond Dogs, Turquoise Days:
In Diamond Dogs, Trintignant is hired by Roland Childe to go on an expedition to visit a mysterious alien structure known as the Blood Spire. Trintignant transforms Childe and Richard Swift into sleek-looking mechanical dogs, or "diamond dogs".
Although there's also Captain John Brannigan, who visited the Pattern Jugglers at one point after being infected by a plague that overtook his nanotechnology implants. However, he essentially turned into a ship, nothing like a dog.
I've included additional examples here, but I believe my first examples are probably the ones you're thinking of, but could also have mixed them up with some of the other characters.
For reference, there are five full novels in this universe/setting, some more connected to the rest than others. There's also a collection of short stories and novellas (about a dozen more works), that are sometimes included in collections with other books. Diamond Dogs, Turquoise Days is such an example.