Monoecious, polygamous or dioecious, annual or perennial herbs with latex, glabrous to stellate pubescent with submersed, floating or emergent leaves; in fresh or brackish water. Roots fibrous, septate or aseptate, few to many; from a stout rhizome or stolon at base of stems or lower nodes. Stems ± short (of variable length), fleshy or spare, branched or unbranched erect, or corm-like or stolons (Luronium), rhizome occasionally terminated by tubers (Sagittaria); turions (winter buds) not formed. Leaves basal, alternate or spirally inserted; sessile or petiolate; petioles terete to triangular, mostly considerably longer than the blade, sheathing base, without auricles, intravaginal scales present; blade linear or orbicular to lanceolate, ovate to rhomboid, with or without pellucid dots or lines, margins entire or undulate, apex acute or acuminate, obtuse to round-acute, base without basal lobes and attenuate or otherwise truncate, cordate sagittate or hastate, with parallel primary veins from base of blade to apex and reticulate or transverse secondary veins. Inflorescence scapose, mostly erect, but some floating, forming racemes of panicles by verticillate branching, involucrate umbels in some, without subtending spathe, bracteate, bracts whorled linear, entire, obtuse to acute or membranous acuminate. Flowers hypogynous, perfect or imperfect, subsessile to (long)-pedicellate, perianth actinomorphic of 6 separate elements in 2 whorls, the outer 3 sepal-like, green, or 6 tepals in two whorls, the outer 3 not green, inner structures delicate, caducous, outer elements erect, enclosing flower, then fruit. Stamens free, 6 (in pairs, alternating with petals), 9 (in two whorls: outer whorl of 6, inner whorl of 3); alternatively, many (∞) of these stamens will be densely and irregularly spaced. Anthers 2-locular, basifixed or versatile, tetrasporangiate, dehiscing by longitudinal slits. Fruit types variable, styles/styluli central terminal, ventrally biased terminal (almost dorsal in Luronium natans) or lateral. Achenes 6 (Caldesia), 6+ in untidy spiral (Baldellia), 20 in one ‘whorl’ (Alisma) or ∞ (-100) spirally on orbicular to conical receptacle (Echinodorus, Sagittaria). Schizocarp: 6-10 fused elements (Damasonium). Follicles (Limnocharis) 6-18 in a ring, ± basally adnate. All fruits contain campylotropous (horse shoe-shaped) apotropous ovules, epitrous in Luronium, bitegmic with a macropodous embryo, lacking endosperm at maturity; some seeds glandular (mucilage) (e.g. Butomus). Achenes: one basal campylotropous ovule, few with funicular aril, rarely with operculum. Schizocarp: according to the 6 species 1-2-(6) ovules on marginal placentae ± funicular aril. Follicle: 1-locular, ∞ ovules restricted to veins / vein endings, on lateral sides of follicle, funicular aril noticeable, but weak in Limnocharis and absent from Butomus. Strong dorsal side of follicle free of seeds.