1.) Can I (as a non-resident) register the boat in either Florida or Texas, if so tax-wise which is the better option?
Most state are willing to accept a registration, even if you aren't actually there. You're just sending them money, why say no? However, just because you registered it in one, does not exempt you from the other. Registration requirements are usually triggered by the time you spend in the state. They are all a little different.
2.) Is is possible for a non-resident with no credit history in the USA to get full insurance cover for a boat (38' to 42')?
I would think you could be covered, however, the premium might not be friendly. The thought that you would be away from the boat for long periods will make the insurance company nervous that no one is available to mitigate risks. A contracted captain helps.
3.) Whilst I might have the resources to buy a boat outright, in the eventuality I need finance for say 50% of the boat, does anyone (especially boat vendors/dealers) know how easy that might be to get if I can provide full documentation from a recognised bank about my financial position? Once again - no credit history in the US.
I will bet there is a bank from your home country that has a branch here. Almost all have business here. That's worth exploring. I suspect that with a good downpayment, you will find a local lender that is interested as well.
4.) Lastly, would it be advisable to keep the boat in a marina up north (Annapolis) from June to October and then down to Florida the rest of the year? Would I end up paying tax in both places?
This is no small endeavor. The variance in your insurance premium isn't likely going to cover the cost to move the boat. If you are motivated to preserve the boat and avoid the insurance claim, that makes sense. Whether you will owe local use tax is completely variable, state by state. Again, it typically has to do with how long you stay and they all set different standards.