Floatplanes and flying boats are very different to each other, and in my opinion, should be separate endorsements on a pilot license. Each have their pros and cons, though for safety, I would rather be flying a flying boat than a floatplane. Floatplanes are very much easier to dock!
Flying boats are rugged, and lighter for that ruggedness than a floatplane. As their C of G is lower in the water, they generally have wingtip floats, and a full length hull, they are much more stable on the water, and during mishandling. But, they are also vulnerable to porpoising, and loosing control. Floatplanes can be safely flown with less pitch finesse, but ultimately will bite a careless pilot every bit as hard. Flying boats are much easier to step turn than a floatplane, and keep a bigger margin of safety.
Lakes are variable in the type of water to be encountered, because of size, exposure to winds, fetch and marine traffic. Some lakes I fly into are like a mill pond every time, but they're small. Other lakes are large and can whip up badly and quickly. These variables mean that the pilot must be familiar and aware of all of these factors. There is nearly never the assurance of aerodrome like comforts and infrastructure to make the pilot's life easier. You gotta work it out for yourself, and getting it wrong could be fatal.
Ocean operations have the potential to be different, in that conditions don't change as quickly, but there are hazards uncommon in lakes. Mostly, those hazards are very long period swells, which will make any takeoff or landing run a wild ride, inviting disaster. I've been very tempted by a smooth ocean surface, but only ever landing in protected areas. The main aspect of this is that you will have shoreline as a reference, and importantly, you can watch the the water against shore. Even though you may not see swells in the water, you'll see them against shore, and know there is a hazard there.
That said, harbours and well used waterways are difficult for water flying operations. Boats just don't know what to do around 'planes. The boat/ship wakes not only give you a rough ride, but will superimpose to become horrendous hazards. In some cases, there are designated water aerodrome areas, and this can help, but the rough water can still get you, a charted area does not convince the water! Last summer, approaching Split, Croatia, I was informed that [on a super nice day] the airport had been closed - too much holiday jet traffic for them to handle. So, after some complex arrangements, I was permitted to use the harbour aerodrome. It's a square for ECA's Twin Otters, it was a piece of work to find an area of water calm enough for the 182. I did not want to leave it corroding in the salt water over night, so thankfully, Split airport allowed me in at sunset. I was able to get a fresh water wash for it. Otherwise, every ocean operation I have flown has been followed by a fresh water splash around and landing to rinse the 'plane.
My water flying license to learn began more than 25 years ago, with intense (sometimes annoying) parental type mentoring for the first few hundred hours (they were not my floatplanes, so I had to accept the mentoring to use the 'plane). But, that kept me alive. A thousand plus water hours later, I'm beginning to feel confident on the water - not that I can do anything, but that I can select accurately the conditions I can safely handle in whatever I'm flying.