ı am not trying to split hairs... but for the people learning Turkish from scratch... ıs duo using turtle for kaplumbağa because ıts easier to type than tortoise which is the perfect translation! tortıose ıs totally land based and turtles are a 99% sea swımmers... so are 2 dıfferent anımals. therefore a turtle ıs deniz kaplumbağası
The meanings of these two words are so blurred in English that it isn't too terribly important. I am from Ohio and everyone always talked about the turtles they saw and not the tortoises :) Turkish is the same in this respect that the daily language and scientific language differs.
This is going to be splitting hairs even more, but what about all the little turtles that live in ponds and streams? Do you still call those deniz kaplumbağaları? (They're not tortoises, but they don't live in the sea.....)
In English, we have:
tortoises -- totally land based.
turtles -- amphibious, but some kinds live in fresh water and some kinds in salt water.
sea turtles -- this is what we usually call the turtles who live in the ocean.
But a lot of people confuse tortoises and turtles, and "turtle" is easier. So "turtle" is often used as a generic term for any turtle-like creature.
In Turkish we have "tosbağa", which is totally land based, but it's not used too often. Kaplumbağa is the all-around word for all types. If you so desire, you can specify:
Kara kaplumbağası (tosbağa) = tortoise
Deniz kaplumbağası (caretta caretta?) = sea turtle
Göl kaplumbağası = lake turtle
Gölet kaplumbağası = pond turtle
Irmak kaplumbağası = River turtle
etc. Turks are simple. ;p