Umm… although this is a little off-topic I would like to add on the differences between technologies on different islands a bit.
Basically only in the grandline are there extreme differences on different islands, as aposed to the east north south and west blues. Remember that all of the different islands in the Grandline run on different magnetic levels. Those levels can indirectly control the weather and the temperatures of the different islands.
Each islands climate is completly different because of that one variable, therefore technology is also changed. Basically that weather controls how crops are grown. If there is a way to sustain and cultivate crops for sustinance, then the people who grow them have time to develop new skills. There is also another factor that goes into this, and that is fish.
Since the people of the Odaverse actually live on islands that vary to the size of a house to an actual country, the key similarity is that they have the ablilty to get fish. Fish is brain food, meaning that it can actually help people get smarter, by allowing more brain stimulation via fish oil. With that taking in place, people can learn to think better, thus they learn skills faster. Those skills lead up to battle ships, dials, weaponry, machines ect...
Remember since cultivation on different islands of the grandline are totally totally different, the way that an islands technology is formed is also very different. Why is it that most islands in the grandline that we see in one peice have the same technology? There's a very simple answer, and it's called colonization.
These islands are then colonized by different representatives, which we can assume is the WG. The WG pretty much has the control over the different islands of the grandline, thus we have technological advances brought over from HQ to these islands. After that all of those colonized islands are now connected to one another through that one source of the WG. There the technology because the same, because of the same source.
The one thing that has really beffuttled me in the grandline, is the Poneglyphs. It would be next to impossible to have all of the poneglyphs to have been made from a substance that is unknown, and yet exactly the same as all other poneglyphs. This is literally an impossiblities, because it's like gold in our world. Gold ore is a valuble ore that is only found in certain places, due to the way the minerals are formed through the Earth. You cannot find it just by digging in the sand. Only specific places have the conditions for gold to be formed, and by luck, we would find exact places like that
How would it be possible to find this X mineral all over the grandline?
My theory is, either the poneglyphs really were seperated all of over the grandline via some way of transportation or, the would was shaped exactly as our planet was, millions of years ago, where we were just this huge hunk of land that had split into different continents. I think that the 200 years of lost history would have been the seperation of the world and the islands, and that all history that prior to this event could have also been a fabricated lie created from the WG, and that the Ancient weapons caused seperation