Yeah they look like leeches and if they are attaching themselves to you and drinking blood, that confirms it.
Try and avoid letting them bite you. You shouldn't catch anything from them biting you but there is always a chance, especially if they are in a polluted water way.
Salt water tends to kill them so if you find them on plants or ornaments, put them in salt water for a few minutes until they drop off and sink to the bottom.
If they get on your skin, you can light a match and burn them off or try to pull them off. They use an anti-coagulant to stop the wound from healing so you will bleed for a bit after removing them.