i,d stay in ranong rather than kothaung(theres not alot to it),alot cheaper,and much more comfortable
The crossing to ko thaung is 2/3miles from ranong itself but you should be able to find some burmese touts on the thai side(alot of people used to cross for a day to renew their thai visa) who will tell you about the fast boat north-If youve time try and find a tout you trust and knows burma not just ko thaung,pick his brains and build a relationship,anything you pay him,which isnt alot, usually is worth it tenfold for the info you,ll get.
Also its a good idea to buy a gsm sim,and pick up the numbers of any helpfull burmese you meet to help you translate(alot of places very few people talk english and last time I was in myeik I was an attraction because alot of people hadnt met a european(i didnt see any while there-maybe different now)
If you go through maisot its a long long detour south and the road into burma from maisot isnt comfortable--there was a plane from bangkok to ranong,if not surat thani and bus--thai roads are alot more comfortable than burmese.
change money into $ in bangkok, make sure the notes are clean and unfolded and try to keep them that way-$50,s are best.I think the fast boat from kothaung only takes $ from you.The islands you pass on the boat are nice and worth seeing.
I know that you could not go by road north from myeik,you had to fly.
The only problem with less touristic places is they generally have limited electricity/infrastructure,it sounds ok until your trapped in a mosquito net 35c gasping for breath and pouring with sweat and have to keep going to the toilet in the dark because the food you ate had gone off in the heat.?--take 2 packs of norfoxacin(you can buy it over the counter)
Try and avoid trains,buses are more comfortable and usually quicker,boats are more comfortable as long as you pay for better class.
Like Ive said,it often pays to give someone some cash to show you the way around,don,t trust them too much,but for $10/$20 they can show you the way around places like yangon /mandalay-best places to eat,where to make cheap calls home from,buses/prices countless things you,d never have time to find out by yourself.
Burma is alot bigger than you think,and poor roads etc make traveling slower,in alot of places you are going back 50/60 years.