The Tideless Baltic is an extensive and enjoyable cruising area with many hundreds of islands and anchorages to explore in largely fresh water, and a wide variety of countries and cultures. The sailing season is short, 3 months, maybe 4, depending how robust you are, from the end of May (when the water is very cold) to the beginning of September. Really long days in June and July are a tremendous bonus, when periods of fine dry weather and brilliant visibility are interrupted by occasional remnants of Atlantic depressions. Luckily these have dumped much of their moisture before arriving.
NW Baltic. One of the major attractions of the NW Baltic (Denmark, Sweden, the Gulf of Bothnia and the Finnish coasts) is pilotage among the vast archipelagos, lightly populated, and rarely crowded. However, in these the water is flat, whatever the wind, and there are large numbers of safe, attractive and free anchorages. Cruising also extends inland of Stockholm among the many lakes which, linked up, comprise the Gota canal. Marinas? Plenty, fairly expensive. The cost of living is a little higher than UK (especially for alcohol), but berthing while cruising, with so many free anchorages, can be very much cheaper.
SE Baltic. The south eastern shorelines are completely different; low lying, quiet, cheap, with some lovely, if crumbly, old towns to enjoy and extensive lagoons to explore in (what was) eastern Germany. Cruising in other parts is from marina to marina, and you'll be sampling interesting countries recovering from the old communist yoke.
There are very good facilities for yachts, but laying up for winter should anticipate temperatures well below freezing - and very long nights. In the sailing season:
Heating should be considered, as should wet suits for early season underwater inspections.
In relatively tideless waters, sewage disposal is regulated. Holding tanks, although not required, will be needed, with pump out fittings.
Mooring, with only small variations in water level, is usually end-on, either with your own ground tackle, or picking up a buoy, and often with the shore end attached to rocks. Ingenious bits of equipment make end-on mooring easier.
Mosquitos are a bore.
Baltic Entries. The most commonly used entrance to the Baltic is the Nord-Ostsee canal, a rather boring big ship canal about 60nm long, joining the Elbe estuary to Kiel, a popular yacht charter centre. Alternative entries are into the Skaggerak, passing north of Denmark, or hit further south to spend a few days wandering through the lakes and waterways of north Jutland (north Denmark).
From UK. From UK there are broadly three options. The soft option is to travel from Dover to France, deciding either to coast hop northwards, or take a mast up inland route through the Netherlands. This can be a cruise objective in its own right, occupying two to four weeks. The stiffer option is to cross from East Anglia to the Netherlands, a 24 hour or so passage, with some busy traffic lanes to cross. Crossings from further north are voyages for the adventurous, probably aiming to enter the Skaggerak after 3 or 4 days at sea from North England or Scotland.